[en] Thermochemical data are required for specific fission product and reactor materials compounds in order to quantify the consequences of a severe accident within a light water reactor. Approximately 40 important compounds/systems have been identified for study for which thermodynamic data did not exist or were inadequate. Work is described on the analysis of approx. half these systems. Experimental studies have been undertaken to compounds: Cs₂MoO₄(g), CsBO₂(g), Cs₂RuO₄(c), Cs₂RuO₄(g), Cs₂MnO₄(c), Cs₂CrO₄(g), Cs₂TeO₃(g), Cs₂Te(g), InI(g), InI₃(g), In₂I₆(g), In₂Te(g), Cd(OH)₂(c), Cd(OH)₂(g), TeO(OH)₂(g), CdI₂(g), Cd₂I₄(g), Cs₂CdI₄(c), CsCdI₃(g), Cs₂CdI₄(g)Cs₃PO₄(c) and Cd-In-Ag. Critical assessments have been made on the following systems: In-I, In-Te, Cd-I, Sr-B-O and Ba-B-O. The thermodynamic quantities of these compounds have been calculated over the temperature range from 298 to 3000 K. The adoption of these data within appropriate modelling codes will allow the fission product species and transport to be predicted with greater confidence, thus providing more accurate assessments of the consequences of severe reactor accidents. (author). 69 refs.; 36 figs.; 46 tabs