Positive ion compositions from one mid-latitude and four high-latitude rocket flights were used to infer the H2O, CH4, H2CO, H2O2, and HCN concentrations in the mesosphere. The water vapor mixing ratio was highly variable between 0.1 and 5 ppmv. Only partial agreement is obtained with the ground-based microwave measurements and/or model prediction by Garcia and Solomon (1985). The observed methane concentration varied between 70 and 4 ppb in daytime and 900 and 100 ppbv at night, respectively. Unexpectedly high H2CO concentrations have been determined with 2CO>/2O> ratios between 6¿10-4 and 0.1. Additional CH4 oxidation in heterogeneous chemical processes or an unknown direct local formaldehyde production is proposed to account for the high H2CO concentration. A mean HCN volume mixing ratio of 6¿10-10 was inferred. A local source, CH3+N→HCN+H2, may explain the HCN increase from 0.15 ppb in the stratosphere to 0.6 ppb in the auroral mesosphere.