Abstract: A state, particularly a disease state, associated with the production of volatiles is detected by passing a sample containing the volatiles to a single sensor. This may be a semiconductor gas sensor element or a surface acoustic wave device. This provides an output signal, e.g. in the form of a tailing peak. A plurality of characteristics of the signal (e.g. peak height and maximum positive gradient) are measured to characterise the sample and hence the underlying state. For example we can discriminate between urine samples which are (a) infected with proteus, (b) infected with E. coli or (c) uninfected.