Abstract: A method of identifying an analyte, using a chemical sensor (e.g., a piezoelectric crystal oscillator) which produces an output frequency responsive to different analytes by a characteristic change in the output frequency. The sensor is exposed to a step change (to) in concentration of the analyte and the resulting frequency characteristic measured. An expression for this characteristic is derived consisting of two exponential functions (F, S), one (F) derived from the early part of the frequency characteristic and the other (S) from the latter part. The four parameters (A.sub.2, A.sub.3, A.sub.4, A.sub.5) resulting from the functions so obtained are characteristic of the particular analyte.