Let be a non-integer. We consider -expansions of the form , where the digits are generated by means of a Borel map defined on . We show that has a unique mixing measure of maximal entropy with marginal measure an infinite convolution of Bernoulli measures. Furthermore, under the measure the digits form a uniform Bernoulli process. In case 1 has a finite greedy expansion with positive coefficients, the measure of maximal entropy is Markov. We also discuss the uniqueness of -expansions.