This paper presents the results of an experimental study of evaporating sessile drops in a controlled environment. The experimental setup allowed the investigation of the evaporation rate of sessile drops under reduced pressure (40 to 1000 mbar) and various ambient gases. Sessile drops of initial volume 2.5μL are deposited on substrates and left to evaporate in a controlled atmosphere. The effect of reducing pressure on the evaporation rate as well as changing the ambient gas is studied. Three different gases are used; namely Helium, Nitrogen and Carbon Dioxide. The role of vapour diffusion as a limiting mechanism for evaporation is studied. It is found that in all cases the evaporation rate is limited by the mass diffusion in the ambient gas provided that interfacial conditions are properly accounted for. This includes important evaporative cooling observed at higher evaporation rates and lower substrate thermal conductivity.