Formaldehyde (HCHO) is an important air pollutant from both an atmospheric chemistry and human health standpoint. This study uses an instrumented photochemical Air Quality Model, CMAQ-DDM, to identify the sensitivity of HCHO concentrations across the United States (U.S.) to major source types and hydrocarbon speciation. In July, biogenic sources of hydrocarbons contribute the most (92% of total hydrocarbon sensitivity), split between isoprene and other alkenes. Among anthropogenic sources, mobile sources of hydrocarbons and nitrogen oxides (NO x) dominate. In January, HCHO is more sensitive to anthropogenic hydrocarbons than biogenic sources, especially mobile sources and residential wood combustion (36% of national hydrocarbon sensitivity). While ozone (O3) is three times more sensitive to NO x than hydrocarbons across most areas of the U.S., HCHO is six times more sensitive to hydrocarbons than NO x, largely due to sensitivity to biogenic precursors and the importance of low-NO x chemistry. In winter, both HCHO and O3 show negative sensitivity to NO x (increases with the removal of NO x), although O3 increases are larger. Relative sensitivities do not change substantially across different regions of the country.