The tertiary 1-(phosphino)thioformamides, Ph2PC(S)NR1R2 (1: R' = R2 = Me; 2: R' = Me, R2 = Ph), are prepared by reaction of N,N-disubstituted thiocarbamoyl chlorides with alkali phosphides. The linkage-isomeric I-(phosphino)thioformimidates, Ph2PC(NR')SR2 (3: R' = R2 = Me; 4: R' = Me, R2 = Et; 5: R' = Ph, R2 = Me), are obtained by alkylation of secondary thioamides. Sulfuration of 1 -5 in boiling benzene yields the corresponding P = S derivatives 6 -10. The cis-and trans-N-alkyl groups are assigned by NMR spectroscopy according to their chemical shift and coupling with phosphorus; the position of the cis-P alkyl signal is strongly affected by the bonding state of phosphorus. The compounds 2 -5 uniformly adopt the Z configuration. The phosphane sulfides 7 -10 form mixtures of E/Z isomers in solution and prefer the Z configuration as a rule. The free enthalpies of activation AG * of the E/Z isomerisation