The solvent swelling of a maturation series consisting of 5 Type I kerogens from the Uinta basin has been studied. The swelling of the kerogens in 10 solvents has been used to determine the solubility parameter (delta) of the samples. It is 9.5-10.0 (cal/cm(3))(1/2). The same data have been used to calculate the number average molecular weights between cross-links (reciprocal of the cross-link density) for the samples. The cross-linking increases during early maturation, and sharply above 80% maturation, but remains about constant from 20% to 80% maturation. Oil formation involves a reconstruction of the kerogen during which bond cleavage to form bitumen is accompanied by bond making to increase the remaining kerogen cross-link density. The change in cross-link density closely parallels a model for maturation in this basin published previously by Sweeney et al.