The Ca2+ concentration and hydrolysis degree [-COO-] dependence of the self-complexation of partially hydrolyzed poly(acrylamide) (HPAM) chains in CaCl2 aqueous solution was systematically investigated by a combination of static and dynamic laser light scattering. We have, for the first time, revealed a transition between the intrachain and interchain complexations. For each given HPAM sample, there exists a critical Ca2+ concentration ([Ca2+](agg)) at which the interchain HPAM complexation becomes dominant. [Ca2+](agg) is related to [-COO-] by [Ca2+](agg) = 7.46 x 10(-9) [-COO-](-1.4), indicating that the complexation is not stoichiomeric and many Ca2+ ions are free in water. We also found that even at [Ca2+] > [Ca2+](agg), the complexation at the initial stage was mainly an intrachain process, but gradually evolved into an interchain aggregation. The length of the initial stage increases as [-COO-] and [Ca2+] decrease. Our results showed that in the complexation process, the weight average molecular weight (M-w) of the HPAM/Ca2+ complexes is scaled to the size (R) of the complexes as M-w proportional to R2.11+/-0.04 for different [Ca2+] and [-COO-], suggesting that the HPAM/Ca2+ complexes have a fractal structure. The fractal dimension of 2.11 +/- 0.04 shows that the complexation is a reaction-limited cluster aggregation (RLCA) process.