The electronic transport properties of ZnSe layers grown by MBE on GaAs sub- strates are studied by small-signal ac admittance, dc current-voltage, and Hall effect measurements. This work is supplemented by a study of TEM and x-ray rocking curve data. We find that the transport characteristics are strongly affected by the proper- ties of the ZnSe/GaAs interface. From the dc and ac measurements, we determine the total barrier height at the interface of thick (1-6 µm) ZnSe layers on n+-GaAs, and find that it is in general voltage dependent. While some samples are found to have a very high peak mobility (> 10,000cm2 /Vsec), an anomalous reduction in the mobility in a large fraction of the samples is found, and attributed to the presence of non- uniform space charge regions.