The flux pinning behaviour of (Nd1/3Sm1/3Gd1/3)Ba2Cu3O7- samples, having different concentrations x of Gd2BaCuO5 secondary-phase nanoparticles, was studied using magnetoresistivity data collected in the presence of 0, 0.5, 1, 2, 4, 6, 8T magnetic fields (H) along and perpendicular to the c axis, with the transport current lying in the ab plane. Significant enhancement of the flux pinning and hence also the irreversibility field Hirr(T) was found. Hirr(T) extrapolated to 77K, Hirr77K(T), was determined from the Hirr(T) plots using best fits to an empirical formula, Hirr(T) = Hirr(0)(1- T/Tc)a, where a is a constant. Hirr77K(T) for &H\parallel c ; axis (a)remains almost constant up to x = 0.2, (b)increases beyond x = 0.2, (c)peaks at x = 0.4 and (d)falls beyond x = 0.4. Further, Hirr77K(T) values are above 15T, the maximum being 243T for the 40wt% sample.