The relative spectral response of a commercially available inductively coupled argon plasma (ICP) emission spectrometer has been determined over a wide spectral range (approx. 190 to >900 nm) using overlapping sets of radiative branching ratios of several atomic and ionic species. Response curves were determined in two ways. In the first, calibrations were based on Ar II and Ar I lines emitted by Ar-filled hollow-cathode lamps used as line sources instead of the plasma torch. In the second, the ICP emission of selected lines of Ni and Fe was used. Branching ratios determined from the ICP emission of lines of Fe I, Se I, and Te I, using Ar lines for the intensity calibrations, were compared with previously published branching ratios or f-values for these atoms, and good agreement was found. The calibrations based on Ar II and Ni I were used to measure further branching ratios, and application to the measurement of branching ratios from selected levels of Ge I and Pd I is shown.