The double endor method invented by Cook and Whiffen (1964) has been used to measure the relative signs of the hyperfine interactions of the unpaired hole in Tm2 +(4f13) with (a) the169Tm nucleus, and (b) the19F ligand nuclei in CaF2. The sign of theinteraction (a) is known from theory, hence our measurement determines the sign of theinteraction (b); both components Asand Apare found to be positive. We have shown that information about the relative signs of two hyperfine parameters may be obtained in a double endor experiment only if the endor signals are essentiallytransient, or have a significant transient component. The proportion of transient componentis somewhat adjustable by suitable choice of experimental conditions. Rapid spin diffusiontends to decrease the proportion of transient component and hence tends to frustrate measurements of relative signs of hyperfine parameters. © 1973 IOP Publishing Ltd.