In order to obtain an upper bound for a solution x of Pillai's equation c z −a x = b under some conditions, we need a result on lower bounds for linear forms in the logarithms of two algebraic numbers. We will introduce here some notations. Let α 1 and α 2 be real algebraic numbers with |α 1 | ≥ 1 and |α 2 | ≥ 1.