In vitro bioassays have indicated that haloacetamides and haloacetaldehydes exhibit the highest cytotoxicity among DBP classes. Previous research has focused on their potential formation from the chlorination or chloramination of aliphatic compounds, particularly nonaromatic amino acids, and acetaldehyde. The present work found that acetaldehyde served as a relatively poor precursor for trichloroacetaldehyde and dichloroacetamide, generally the most prevalent of the haloacetaldehydes and haloacetamides, during chlorination or chlorination/chloramination. Using phenolic model compounds, particularly 4-hydroxybenzoic acid, as models for structures in humic substances, we found significantly higher formation of trichloroacetaldehyde and dichloroacetamide from prechlorination followed by chloramination. Evaluation of the stoichiometry of chlorine reactions with 4-hydroxybenzoic acid and several intermediates indicated that seven successive Cl[+1] transfers, faster with chlorination than chloramination, can form 2,3,5,5,6-pentachloro-6-hydroxy-cyclohexa-2-ene-1,4-dione via chlorophenol and chlorobenzoquinone intermediates. Formation of 2,3,5,5,6-pentachloro-6-hydroxy-cyclohexa-2-ene-1,4-dione may serve as a key branching point, with chloramines promoting the formation of dichloroacetamide and chlorination promoting the formation of trichloroacetaldehyde. The behavior of 4-hydroxybenzoic acid with respect to yields of dichloroacetamide and trichloroacetaldehyde during chlorination followed by chloramination was similar to the behavior observed for model humic acids and several surface waters, suggesting that phenolic structures in natural waters may serve as the predominant, and common pool of precursors for haloacetamides and haloacetaldehydes. Experiments with natural waters indicated that the branching point is reached over prechlorine exposures (100-500 mg-min/L) relevant to drinking water utilities using chlorine as a primary disinfectant and chloramines for maintenance of a distribution system residual.