The output EMF of a fine-wire thermocouple element (chromel-alumel, 0–025 mm dia) has been recorded in detail after rapid electrical heating to 25 K above ambient in various gaseous environments. The EMF always decayed exponentially, thus confirming that thermocouple response in such environments may be characterized by a ‘time constant’ (θ). Time constants have been obtained for a variety of Pt Pt −13%Rh elements in static gas environments. The time constant fell with increasing initial temperature differential (> 40 K) at near atmospheric pressure and variation of θ with support wire alignment indicates that this was due to convective heat transfer. For low initial differential (< 40 K), θ rose with falling gas thermal conductivity but was independent of pressure above a few mmHg. Values of τ obtained from a predictive equation due to Melvin did not agree well with experimental results for this pressure range. Below approximately 20 mm Hg the time constant of a 0–019 mm dia (0–0125 mm wire diameter) element rose with falling pressure; it is shown that the rise can be attributed to increasing heat-transfer resistance due to accommodation effects.