A novel gas sensor is fabricated on the basis of simultaneous measurements of resistance changes (Delta R) and mass changes (Delta M) of a conducting polypyrrole (PP) film assembled on a piezoelectric crystal. Molecular species such as acetone, methanol and ethanol can be recognized by the (Delta R/R(0))/(Delta M/M(0)) values, where R(0) and M(0) are the initial resistance and weight of the PP film respectively. In contrast, the quantitative analysis of the vapour molecules can be achieved by measuring the Delta R/R(0) or Delta M/M(0) values. For a series of primary alcohols the homologues can also be recognized by the simultaneous measurement of Delta R and Delta M. Moreover, it is shown that this sensor is usable to clarify the interactions between the PP film and the vapour molecules.