Elec. properties of seated-model thermal cells in the Li(Fe)-FeS2 system with 78.4% (LiCl-KCl-RbCl)(eutectic)-1.6%LiF as electrolyte were examd. in relation to graphite (4.5 or 9%) and MgO (4.5 or 9%) content in the cathode material. The cells were discharged over 655-905 K; working voltage rose as temp. was raised. At higher C, the working voltage was higher; at 9%C, 1.4 V was achieved at 655 K (vs. the 680 K at 0%C). At 755-805 K, the elec. capacity was max. With C alone (no MgO) as additive, the voltage characteristics was most stable.