Viscoelasticity of a rubber toughened poly(methyl methacrylate) has bern investigated bell in the linear and the non-linear range by means of tensile creep and tensile constant strain rate tests. An empirical time-temperature-strain reduction scheme is proposed Lo analyze tile modulus data obtained at different temperatures and strain levels : a single master curve and relevant lime-temperature and time-strain shift factors have been obtained. Two different thermal histories were considered for testing. It lias been shown that, once aging effects can be considered under conditions where aging time elapsed before testing is the same, modulus master curves obtained from linear and from non-linear data substantially coincide. This shows that both temperature and strain level determine the modulus master curve ria their influence on a single variable : the free volume of the material.