Graphene, a two-dimensional material with exceptional electronic and mechanical properties, is a promising candidate for chemical vapor sensors. However, conventional nanolithographic processes often leave a resist residue on graphene, which degrades its electronic properties and masks its intrinsic sensor responses. This study demonstrates that the contamination layer chemically dopes the graphene, enhances carrier scattering, and acts as an absorbent layer that concentrates analyte molecules at the graphene surface, thereby enhancing the sensor response. A cleaning process was developed to remove the contamination, allowing the intrinsic chemical responses of graphene to be measured. The study measured the structural and electron transport properties of a graphene field effect transistor (FET) immediately after mechanical exfoliation, after contact fabrication using electron beam lithography (EBL), and after a cleaning process. The results showed that the contamination layer significantly affected the transport properties and vapor sensor responses of the device. After cleaning, the device showed a reduction in doped carrier concentration, increased carrier mobility, and weaker electrical response upon exposure to chemical vapors. The study also found that the cleaning process significantly improved the structural and electronic properties of the graphene. The sensor responses to various vapors, including water, nonanal, octanoic acid, and trimethylamine, were measured before and after cleaning. The cleaning process removed the resist residue, enabling the measurement of the intrinsic responses of the graphene device. The sensor responses showed a power law dependence on concentration, consistent with previous studies on metal oxide and conducting polymer nanowire vapor sensors. The results demonstrate that graphene vapor sensors have desirable characteristics, such as rapid response and recovery, and confirm the promise of graphene for this application. The study highlights the importance of cleaning the graphene surface before functionalization to control its chemical affinity. The cleaning process demonstrated here should enable the ready transfer of surface chemistry modifications previously applied to carbon nanotubes for targeted molecular sensing in the vapor and liquid phases.Graphene, a two-dimensional material with exceptional electronic and mechanical properties, is a promising candidate for chemical vapor sensors. However, conventional nanolithographic processes often leave a resist residue on graphene, which degrades its electronic properties and masks its intrinsic sensor responses. This study demonstrates that the contamination layer chemically dopes the graphene, enhances carrier scattering, and acts as an absorbent layer that concentrates analyte molecules at the graphene surface, thereby enhancing the sensor response. A cleaning process was developed to remove the contamination, allowing the intrinsic chemical responses of graphene to be measured. The study measured the structural and electron transport properties of a graphene field effect transistor (FET) immediately after mechanical exfoliation, after contact fabrication using electron beam lithography (EBL), and after a cleaning process. The results showed that the contamination layer significantly affected the transport properties and vapor sensor responses of the device. After cleaning, the device showed a reduction in doped carrier concentration, increased carrier mobility, and weaker electrical response upon exposure to chemical vapors. The study also found that the cleaning process significantly improved the structural and electronic properties of the graphene. The sensor responses to various vapors, including water, nonanal, octanoic acid, and trimethylamine, were measured before and after cleaning. The cleaning process removed the resist residue, enabling the measurement of the intrinsic responses of the graphene device. The sensor responses showed a power law dependence on concentration, consistent with previous studies on metal oxide and conducting polymer nanowire vapor sensors. The results demonstrate that graphene vapor sensors have desirable characteristics, such as rapid response and recovery, and confirm the promise of graphene for this application. The study highlights the importance of cleaning the graphene surface before functionalization to control its chemical affinity. The cleaning process demonstrated here should enable the ready transfer of surface chemistry modifications previously applied to carbon nanotubes for targeted molecular sensing in the vapor and liquid phases.