Small angle x-ray scattering (SAXS) measurements are used to quantify the wavelength and amplitude of the sidewall roughness in a lithographic line:space pattern due to vertical standing waves present during the photoresist exposure. Analytic equations are derived to model the x-ray scattering intensity and are used to determine the periodicity and amplitude of the standing wave roughness. The average periodicity, or pitch, and the linewidth were L=422±1 nm and w0 =148±1 nm. The period and amplitude of the standing wave roughness were λs =65±1 nm and As =3.0±0.5 nm. These results demonstrate the potential of SAXS measurements to quantify nondestructively and quantitatively dimensional deviations from an ideal structure.