gamma-Fe2O3/SiO2 nanocomposites have been prepared using a sol-gel procedure, starting from iron nitrate and triethyl orthosilicate. The addition of acids to the sols resulted in a way to increase particle size, keeping low iron concentrations, and to narrow the particle size distribution of gamma-Fe2O3 in the glass composite. Thus, the addition of 0.56 mmol HCl (approximate to0.1 M) to a solution having an Fe/Si molar ratio of 18% leads to an increase of gamma-Fe2O3 particle size from 6 to 13 nm together with a remarkable decrease in the polydispersity degree of the particle size from 66% to 15%. The iron oxide crystalline phase, the particle size and shape, and the homogeneity of the resulting nanocomposites were studied by X-ray diffraction, Mossbauer spectroscopy, and transmission electron microscopy. Si-29 NMR and Fourier transform infrared techniques were used to study the changes in matrix structure due to the presence of acids. The influence of acid addition on the size of the resulting magnetic particles was found to depend on the matrix microstructure, the charge environment, and the presence of the counteranions of the acids. Magnetization studies at room temperature showed superparamagnetic behavior for all the nanocomposites, except for the sample with the largest particle size (13 nm), which exhibits ferromagnetism. Optical measurements showed that as the size of the magnetic particle decreases, a blue shift in the UV-vis absorption spectra occurs, which is related to quantum confinement effects.