AbstractTo complete results presented in this and in previous papers of this series as well as published in patents of other authors a review is given on known and new variations of the heterocyclic moiety in GS 13005 type thio‐ and dithiophosphoric acid esters (1, 2) by modification of the 1,3,4‐thiadiazol‐2(3H)‐one ring 5 and by its replacement by analogue five‐ and homologue six‐membered rings.Among new esters of this type some containing the pyrazolinone ring 3 or a 2‐alkoxy‐4H, 6H‐1,3,4‐thiadiazin‐5‐one ring 10 (homologue of the original 5‐methoxy‐1,3,4‐thiadiazol‐2(3H)‐one ring in GS 13005) show no remarkable pesticidal activity, some others containing a pyrazolering 7 or a 3(2H)‐pyridazinonc ring 8 are moderately to highly active but toxic to inauinials in the same proportion.Attempts to prepare seven‐membered 2‐alkoxy‐ and 2‐alkylthio‐6,7‐dihydro‐l, 3,4‐thiadiazepin‐S(4H)‐ones 11, Z‐rnethoxy‐l,3,4‐thiadiazepin‐S(4H)‐one 12 (ring vinylogue of the original 5‐methoxy‐l,3,4‐thiadIazol‐2(3H)‐one ring in GS 13005) and its 7‐methyl‐derivative have been unsuccessful due to unexpected side reactions, such as: five‐ring closure of 3‐(3‐chloropropionyl)‐thio‐ and ‐dithiocarbazic acid esters 22 to pyrazolidinone derivatives 23, pyrazolinone ring closure of a 3‐(acetoacety1)‐thiocarbazic acid O‐methyl ester derivative 26, bromine attack on sulfur in 3‐(2‐alkenoyl)‐thiocarbazic O‐methyl esters 29 instead of bromine addition at the double bond, and halogen splitting off without ring closure in 3‐(2,3‐dihalogeno‐alkanoyl) ‐thiocarbazic O‐methyl esters 30 prepared by acylation of thiocarbazic acid O‐methyl ester with dihalogeno‐alkanoyl‐chlorides.