Using poly(acrylic acid) (PAA) and polyacrylamide (PAAm) as ail example, we studied the mechanism of hydrogen-bonding complexation in dilute solution by both static and dynamic light scattering. In 20 mM phosphate buffer at pH = 3, the condition suitable for complexation of PAA and PAAm, neither PAA nor PAAm stayed as individual polymer chains at 8.0 mg/mL. Instead, most of PAA formed "multimacroion clusters" (or slow mode) due to the negative charges, and a small portion of PAAm formed associates mainly via hydrogen bonds. After PAA and PAAm solutions were mixed at stoichiometric ratio at room temperature, the slow mode, with PAA being dominant, persisted. The hydrogen-bonding complexation induced by cooling was evolved from the slow mode rather than from the single PAA or PAAm chains. Therefore, at the temperature where phase separation occurred, a fairly large amount of free PAAm chains still existed in the system. Our study demonstrated that the growth of the hydrogen-bonding complex, as well as the optimal ratio for complex, was highly dependent on the status of the component polymers before complexation.