Abstract The suitability of ion-selective electrode for the determination of ammonia in pyrolysis gases of fossil fuels was studied. The ammonia was absorbed into acidic solution and two kinds of determination methods were carried out. The ammonia was either measured directly from the acid solution, or ammonia was first released into the gas phase and then determined (air gap method) by the ammonia selective electrode. The electrode functioned well in both cases, but the linear calibration range was rather narrow, slightly more than one tenfold. The quantitative detection limit in the water phase was 5 × 10 −6 M (0.085 ppm) NH 3 and in gas phase operation solutions above 5 × 10 −4 M (8.5 ppm) NH 3 it was possible to measure quantitatively. The applications were carried out with Finnish energy peat samples and a coal sample.