The intrinsic response of graphene vapor sensors is significantly impacted by the contamination layer left by conventional nanolithographic processing. This contamination layer chemically dopes graphene, enhances carrier scattering, and acts as an absorbent layer that concentrates analyte molecules at the graphene surface, thereby enhancing the sensor response. The authors demonstrate a cleaning process that effectively removes this contamination, allowing the intrinsic chemical responses of graphene to be measured. The cleaned device shows improved electronic properties, including reduced carrier doping, increased carrier mobility, and weaker electrical response to chemical vapors. The cleaning process also enables the measurement of the intrinsic sensor responses to various vapors, such as water, nonanal, octanoic acid, and trimethylamine, which are not intrinsic to the graphene due to the contamination layer. These findings highlight the importance of cleaning in achieving the full potential of graphene vapor sensors and suggest that graphene devices can be effectively functionalized for targeted molecular sensing in both vapor and liquid phases.The intrinsic response of graphene vapor sensors is significantly impacted by the contamination layer left by conventional nanolithographic processing. This contamination layer chemically dopes graphene, enhances carrier scattering, and acts as an absorbent layer that concentrates analyte molecules at the graphene surface, thereby enhancing the sensor response. The authors demonstrate a cleaning process that effectively removes this contamination, allowing the intrinsic chemical responses of graphene to be measured. The cleaned device shows improved electronic properties, including reduced carrier doping, increased carrier mobility, and weaker electrical response to chemical vapors. The cleaning process also enables the measurement of the intrinsic sensor responses to various vapors, such as water, nonanal, octanoic acid, and trimethylamine, which are not intrinsic to the graphene due to the contamination layer. These findings highlight the importance of cleaning in achieving the full potential of graphene vapor sensors and suggest that graphene devices can be effectively functionalized for targeted molecular sensing in both vapor and liquid phases.