The reaction of dimethyl-phosphine and -arsine with tetrakistrifluoromethyl-diphosphine and -diarsine gave the mixed fluoroalkyl-alkyl derivatives (CF3)2P·P(CH3)2, (CF3)2As·P(CH3)2, (CF3)2P·As(CH3)2, and (CF3)2As·As(CH3)2 containing P–P, As–P, and As–As bonds. The reactions appear to depend on the basic strength of the hydride since the fully fluorinated phosphino-arsine (CF3)2P·As(CF3)2 was obtained from the action of bistrifluoromethylarsine on tetrakistrifluoromethyldiphosphine but the diarsine was not affected by bistrifluoromethylphosphine. The fully fluorinated phosphino-arsine, also obtained from the reaction of a mixture of bistrifluoromethyliodo-phosphine and -arsine with mercury, could not be isolated from the diphosphorus and diarsenic compounds and was characterised spectroscopically. Interesting features of the n.m.r. and mass spectra of the compounds are discussed.