Reaction of the phosphonated hydrazone (2-hydrazineylidenepropyl) diphenylphosphine oxide 1 with p-fluorophenyl-isothiocyanate yields as major product the thiosemicarbazone Ph2P(=O)CH2{C=N-NH(C=S)-N(H)C6H4F}CH3 (2-(1-(diphenylphosphoryl)propan-2-ylidene)-N-(4-fluorophenyl)hydrazine-1-carbothioamide) 2 along with bis(N-p-fluorophenylthiourea) 3 as minor product. This latter product 3 was isolated as main product by direct treatment of p-FC6H4N=C=S with hydrazine in a 2:1 ratio. Both 2 and 3 were characterized by NMR. Furthermore, the molecular structure of 3 was elucidated by an X-ray diffraction study. A conformational DFT study, at the B3LYP/6311 G++ (d, p) level of theo-ry, confirmed a good match between the calculated structure and the experimental one