The implementation of a pH-ISFET (ion-selective field effect transistor) planar chip in a flow-injection system by means of a specially designed flow cell and the application of the sensor to the analysis of volatile compounds is described. Ammonia and sulfur dioxide are determined by exploiting the advantages of the flow techniques combined with continuous gas diffusion. Initial species are converted to gaseous compounds, which diffuse through a microporous hydrophobic membrane into an ammonium chloride or hydrogen sulfite recipient stream, producing a pH variation that is monitored by the semiconductor detector. The linear working range extends from 1 × 10−2 to 1 × 10−4 M for the ammonium system and from 4 × 10−3 to 7 × 10−5 for the sulfur dioxide system, with relative standard deviation of about 1 and 0.5%, respectively.