The construction and field operation of a portable differential psychrometer system is described. Portability is provided by means of a dc powered ventilation system. Examples of temperature and vapor pressure gradients measured with the apparatus are given. Despite small gradients (−0.01°C m−1; −1 Pa m−1), Bowen ratios calculated from the data tend to be precise, demonstrating characteristics like those observed elsewhere. It is concluded that the instrumentation expands the scope of differential psychrometry in evaporation studies and other aspects of boundary-layer work.