A copolymer of 2-(9-anthrylmethoxy)ethyl methacrylate with methyl methacrylate (M(w) = 3.1 x 10(5), anthryl content = 0.8 mol %) and a low-mass model compound were studied with respect to their photodimerization or photo-cross-linking processes in tetrahydrofuran solution. The number of dimers or cross-links formed by ultraviolet (UV) irradiation was determined by UV absorption spectroscopy. A kinetic study on the model compound indicated the existence of a small amount of side reaction, which should be taken into consideration for a quantitative analysis. The concentration dependence of the initial quantum yield of photo-cross-linking of the polymer suggested that, below the overlap concentration c*, the intramolecular cross-linking predominates, while at c > c*, the contribution of intermolecular cross-linking increases with increasing c. This was directly confirmed by the light scattering measurements of the molecular weight and radius of gyration of the irradiated polymer. For a 10 wt % solution (c/c* approximate to 4), the critical number of cross-links, L(g), of gelation was determined to be about 1.0 per chain, of which the intermolecular contribution is estimated to be similar to 0.33. This value is fairly close to the Flory-Stockmayer theoretical prediction (about 0.24).