Results from magnetic viscosity experiments on chemically synthesized magnetic nanoparticles are presented. Our synthesized Fe20Pt80 particles are distinguished by a narrow size distribution but display features associated with broad distributions of activation rates. Results are presented from viscosity measurements in which the remnant magnetization is measured at different times after the removal of a static field, and studied as a function of temperature. We discuss how these results may be analyzed in terms of activation rate distributions for thermal activated reversal processes after the application and removal of a field. A method for the extraction of energy barrier distributions from directly measured time dependent data is presented.