Determination of the wet-bulb temperature at the surface of a material is the basis of one class of humidity measuring instruments, and is important in industrial applications such as dryer modelling and simulation. The psychrometer equation is a frequently used method of estimating wet-bulb temperature, and contains a psychrometer "constant". Analysis shows that this is in fact a variable coefficient affected by temperature, pressure, radiation and conduction effects, and the identity of the gas and vapour. Radiation and conduction affect the difference between adiabatic saturation temperature and indicated wet-bulb temperature. Inconsistencies in currently recommended values for the psychrometer coefficient, including published international standards, are identified and explained. Particular problems arise when the enhancement factor is applied to vapour pressure to account for non-ideality of gases. Special considerations are also needed for-wet-bulb temperatures approaching the boiling point, where the psychrometer coefficient tends to zero. Self-consistent recommendations recently published in the new British Standard BS1339 are given, which cover both the air-water system and a general vapour-gas system.