Most of the gas sensors are based on resistors with inorganic materials and more rarely on other conductometric devices (diodes or transistors). Conductometric sensors have also been designed with molecular materials. Thus, in 2009, Molecular Semiconductor — Doped-insulator (MSDI) heterojunctions were built around a heterojunction between a molecular semiconductor (MS) and a doped-insulator (DI). The MS must be more conductive than the sublayer to take advantage of the heterojunction. The MS is generally of p-type and DI can be of p-type (p-MSDI) or n-type (n-MSDI) material. The energy barrier at the interface depends on the difference in the charge carrier density in the two layers, leading to a variable extent of the plateau in the current-voltage characteristics, according to materials. We use this new transducer to detect different gases. We studied, in particular, the response of MSDIs to ammonia in a broad range of relative humidity (rh). A n-MSDI exhibits a positive response to ammonia (electron donating species) and a negative response to ozone (oxidizing species). Whereas the only material in contact with the gas is a p-type MS (LuPc₂), these responses are opposite to those of a resistor prepared by only this material (LuPc₂). The better stability toward humidity was obtained with Cu(F₁₆Pc) as n-type sublayer. Thus, for Cu(F₁₆Pc)/LuPc₂ MSDI (50 nm/50 nm), the relative response to NH₃ is almost not affected by the variation of rh in the 20-80 % range.