The effect of diffuse functions on geometry and isotropic hyperfine splitting (hfs) constants of radical anions (FCN.-, HCN.-, HCCH.-, BF3.-, BH3.-, and HBO.-) has been studied at the UQCISD/6-311G(d,p) level. The use of standard diffuse functions (6-311++G(d,p)) provides unreliable geometry and hfs constants for radical anions that are kinetically unstable in the gas phase (HCN.-, HCCH.-, BH3.-, and HBO.-). Unreliable results for these radicals are obtained also enlarging the basis set (6-311++G(2df,p)). The reliability of the results can be assured constructing graphics as a function of the exponent of the diffuse functions and analyzing the population of the singly occupied molecular orbital (SOMO) to establish its nature (valence or diffuse MO). A procedure for computing reliable hfs constants in radical anions is proposed.