The photoluminescence properties of amorphous hydrogenated carbon (a-C:H) films grown using a r.f. plasma have been investigated. It is found that the photoluminescence (PL) spectra exhibit a red shift, accompanied by a sharp decrease in PL efficiency and an increase in electron spin density as the applied r.f. power increases. The results can be interpreted by a model in which the a-C:H films consist of two phases with rr-bonded clusters embedded in an sp(3)-bonded amorphous matrix. The excitation and the recombination of electron hole pairs are believed to take place in the pi-bonded clusters. As a result of the confinement effect of electron hole pairs in these clusters, the cluster size becomes an important parameter in understanding the PL properties of a-C:H films.