For the double oxides and other related double A(x)B(y)C(z) compounds in the system AC-BC, linear correlations have been observed between standard enthalpies of formation of the double compounds A(x)B(y)C(z) from the component compounds AC and BC (Delta H-f,cc(0) in kJ (g-atom)(-1) and values of the sum of the molar fraction enthalpies of the component compounds AC and BC ( = x(AC)Delta H-f(0)(BC), in kJ (g-atom)(-1). In general, the dependence of H-f,cc(0) = f() has a minimum, its branches being described with the help of linear equations (the average deviation from the known Delta H(f,cc)0 +/-47% values was less than +/-4.7% for 121 double compounds in 34 systems). A conclusion has been drawn that these regularities (linear approximation rule-LAR) are characteristic for different types of inorganic double compound.