Hydrocarbon oils have been considered for use as thermal energy storage media in solar thermal electric systems. In systems storing the energy as sensible heat, the energy storage capacity is determined by the specific heat of the oil. Among the oils proposed is Caloria HT-43, a product of Exxon Corporation. Previously reported specific heat data on Caloria HT-43 show values differing by as much as 25 percent. An attempt was made to resolve this discrepancy by measuring the specific heat from 77 to 327/sup 0/C. Results show values from four samples varying by as much as 11 percent. The variations are correlated with differences in average molecular weights and inhomogeneity of the samples. The impact on cost of an 11 percent variation in specific heat is less than 1 percent for oil/rock systems and less than 2 percent for an all-oil system.