Isotherms and free energy variations at equilibrium in the case of fixed one-to-n adsorption (i.e. the average occupation of n adsorption sites by one adsorbed molecule) are calculated for one-dimensional models and their properties are discussed. It proceeds that no first order transition can be expected between a condensed (one-to-n) disordered phase and the ordered (one-to-one) solid phase. Configurational entropy of an adsorbed two-dimensional one-to-n phase is computed numerically during a random sequential filling simulation. Since in this case, the two-dimensional phase is built in a disordered manner, the system can swing abruptly from disordered to ordered state.