The principle of congruence claims that certain thermodynamic and transport properties of a mixture of n-alkanes are the same as those of the pure n-alkane of the mole-fraction-averaged carbon number. We discuss the origin of the principle, and attempts at theoretical justification and validation with respect to experimental data. We demonstrate that the principle applies even in cases where no fundamental justification exists. Whenever the principle applies, prediction of thermodynamic behavior of a mixture of n-alkanes is inherently simpler and usually more accurate than that based on empirical equations of state, molecular interaction parameters, and mixing and combining rules. Our experimental verification serves to pinpoint those properties that can be safely and accurately predicted on the basis of the principle of congruence. The solubility of hydrogen in mixtures of long-chain n-alkanes is used as a prime example.