Minerals of the jarosite group can be a significant environmental sink for hexavalent Cr by substitution of chromate for sulfate. The thermochemistry of the synthetic solid solution between jarosite KFe3(SO4)2(OH)6 and its chromate analog KFe3(CrO4)2(OH)6 was investigated by high-temperature oxide-melt solution calorimetry. The enthalpies of formation (ΔH0f) of the latter, as well as of five intermediate compositions in the series KFe3(S1−zCrzO4)2(OH)6, were determined, where z corresponds to the Cr content. The variation of ΔH0f with Cr content deviates from ideality, and negative enthalpies of mixing between jarosite and its chromate analog are observed, suggesting some ordering of the sulfate/chromate groups in the solid solution. The measured enthalpy of formation from the elements of the end-member KFe3(CrO4)2(OH)6 is ΔH0f = −3762.5 ± 8.0 kJ/mol. In view of this work, and considering literature data, ΔG0f = −3305.5 ± 3.4 kJ/mol, ΔS0f = −1533.6 ± 29.2 J/(mol·K), and S0 = 487.7 ± 29.2 J/(mol·K) are recommended for KFe3(CrO4)2(OH)6.