Proton magnetic relaxation dispersion (PMRD) measurements are made with a view to investigating the influence of an underlying smectic phase, on the isotropic and nematic phases of the binary liquid crystalline mixture of 4DBT and 12CB, at two distinct concentrations, namely 50% 12CB (has no smectic phase) and 70% 12CB (retains S-Ad phase of 12CB). Analyses of the data in terms of relevant molecular processes show that the influence of smectic clusters on the nematic order fluctuations persists even in the isotropic phase. Further, the molecular dynamics are qualitatively different in the respective nematic phases of the two mixtures. (C) 2009 Elsevier B. V. All rights reserved.