Various methods of diversity reception are considered as a means of reducing the errors in mobile radio data systems caused by fading and ignition noise. Data modems designed for subcarrier and direct modulation of the carrier have been built, and both binary f.s.k. and p.s.k. have been used. Three dualdiversity receivers utilising switched diversity, predetection combining and postdetection combining have been implemented, and their effectiveness has been tested in both laboratory tests and field trials at v.h.f. Significant reductions in bit error rate have been achieved.