New entropy functions can be obtained through Legendre transformation on natural variables.Massieu functions are obtained through Legendre transformation on natural variable X, where X can be either P, V, S or T . In the present investigation, generalised Massieu functions are derived using other methods and it is shown that these functions can be represented as a total change in X which is a consequence of relationships among energy potentials. Thus, Massieu functions also follow linear relationships like energy potentials. Variation of energy potentials with natural variables, their partial derivatives, are derived using Massieu functions. It is further shown that variation of energy potentials with natural variables can be represented either as its conjugate or a Massieu function of its conjugate. Massieu entropy function S*_α = PV/T can be used to represent the equation of state. It follows that for an ideal gas, entropy S*_α is constant and equal to R, the universal gas constant for ideal gases. Thus, the ideal gas constant is an entropy term. It is also inferred that Boltzmann’s constant represents entropy S*_α of a single element. It is proved that the change in entropy S*_α during the mixing of distinguishable and indistinguishable ideal gases is zero.