An investigation of the effect of interparticle interaction and particle size distribution has been carried out on iron oxide nanoparticles dispersed into porous Vycor glass. ␥-Fe 2 O 3 nanoparticles dispersed into monoliths of Vycor glass were obtained using impregnation-decomposition cycles through the single-source metallo-organic decomposition process. Magnetic properties were investigated by ac magnetic susceptibility measurements, as a function of temperature at different frequencies, by measuring zero-field-cooled and field-cooled magnetization curves and by constructing hysteresis loops at different temperatures. A log-normal size distribution of monodomain nanoparticles has been deduced from the analysis of the magnetization curves. 57 Fe Mössbauer spectroscopy was also employed for investigating the magnetic behavior as a function of nanoparticle size. The systems exhibit typical superparamagnetic behaviors with a wide particle size distribution that can be changed without significantly affecting the interparticle interaction. The experimental data are discussed in terms of the evolution of the particle size distribution with the number of impregnation-decomposition cycles used for preparing the nanoparticles.