The drift mobility in a-Si p+–i–n+junctions under forward bias has been studied by the voltage pulse technique as a function of d.c. bias. This technique shows a dependence of the drift mobility on d.c. voltage and forward bias current. These results do not agree with recent laser-induced time of flight (TOF) measurements by Goldie, Le Comber and Spear. The voltage pulse technique has been criticised by Goldieet al.as giving incorrect results owing to an increase in capacitance under forward bias. In this Letter we consider more carefully the question of theRCtime constant under forward bias conditions and find that C does not change. Both the voltage pulse and laser-induced TOF techniques employed under forward bias are discussed in an attempt to reconcile the different results.