The effects of wetland treatment on disinfection byproduct precursors were evaluated for six constructed wetlands receiving agricultural return flows in the Central Valley of California. Wetlands varied in size, age, vegetation, hydrologic residence time (0.9-20 days) and water management (continuous flow vs. flood pulse). The effects of wetland treatment were determined by analyzing input and outflow waters for dissolved organic carbon concentration and quality, bromide concentration, and formation potentials for nine disinfection byproduct species, including trihalomethanes, haloacetronitriles, chloral hydrate, and haloketones. We hypothesized that hydraulic residence time was a key factor governing differences in disinfection byproduct precursors. Small wetlands (<3 ha) with short hydraulic residence times (<2 days) did not produce significant changes in disinfection byproduct precursor concentrations with respect to the agricultural return flows input to the wetlands. In these wetlands hydraulic residence times were not long enough to promote processes that adversely affect dissolved organic carbon and bromide quantity, such as evapoconcentration and leaching from vegetation. Thus, less negative effects were associated with disinfection byproduct formation. In contrast, larger wetlands (>100 ha) with long hydraulic residence times (>10 days) resulted in higher dissolved organic carbon and bromide levels, increasing disinfection byproduct formation by factors ranging between 1.7 and 10.2 compared to agricultural return flows. Results from this study provide important information for optimizing the design and management of constructed wetlands to effectively combine control of disinfection byproduct precursors with other water quality parameters.