Lignin-based chemicals, starch, and urea were used as modifiers for phenol-formaldehyde resol resins. The effects of the addition stage of the modifiers used in the synthesis of the resins and the type of modification reagent on the structures of the resins and their molar masses and reactivities were investigated. The modifications with corn starch and lignin promoted condensation; this was verified by increased molar masses and high ratios of methylene bridges to the sum of free ortho and para aromatic groups with respect to the corresponding reference resin without a modification reagent. The later the modifier was added to the resin condensation mixture, the more methylene bridges were formed with respect to the amounts of free ortho and para aromatic groups. In addition, when urea or wheat starch was added in the later condensation stage, the final condensation also reached high stages. The modifications with lignosulfonate and starch, as well as the early addition of urea, enhanced p-pЈ bridge structures. The lowest condensation stage and, therefore, the highest reactivity were found when wheat starch was added with the starting reagents. The curing heat of the wheat-starch-modified resins decreased according to the deferred addition point of starch.