The paper proposes new mechanisms for understanding neutrino masses in superstring models that contain E6-singlet zero-mass fields after compactification. The authors show that the low-energy gauge group of these models can be phenomenologically acceptable. They discuss ΔB = 1 and ΔB = 2 baryon number violating processes in these models, particularly focusing on neutron-anti-neutron oscillation and proton decay. The models use light E6-singlet fields to generate small neutrino masses, and the requirement for specific quantum numbers of light Higgs fields is satisfied for phenomenologically acceptable low-energy gauge groups. The paper presents two detailed models for neutrino masses, Model I and Model II, which involve the use of E6-singlet fields and discrete symmetries to break the gauge symmetry at an intermediate scale. These models avoid catastrophic proton decay and may lead to observable neutron-anti-neutron oscillation under certain conditions. The authors also discuss the phenomenological implications of these models, including the constraints on the discrete symmetries and the nature of the low-energy electroweak gauge group.The paper proposes new mechanisms for understanding neutrino masses in superstring models that contain E6-singlet zero-mass fields after compactification. The authors show that the low-energy gauge group of these models can be phenomenologically acceptable. They discuss ΔB = 1 and ΔB = 2 baryon number violating processes in these models, particularly focusing on neutron-anti-neutron oscillation and proton decay. The models use light E6-singlet fields to generate small neutrino masses, and the requirement for specific quantum numbers of light Higgs fields is satisfied for phenomenologically acceptable low-energy gauge groups. The paper presents two detailed models for neutrino masses, Model I and Model II, which involve the use of E6-singlet fields and discrete symmetries to break the gauge symmetry at an intermediate scale. These models avoid catastrophic proton decay and may lead to observable neutron-anti-neutron oscillation under certain conditions. The authors also discuss the phenomenological implications of these models, including the constraints on the discrete symmetries and the nature of the low-energy electroweak gauge group.