A spectrophotometer operating in the ultraviolet region can be used to obtain accurate and precise silicon dioxide thickness determinations on silicon. Excellent accuracy and precision is obtained by considering: (1) the refractive‐index dispersion of the silicon dioxide; and (2) the theoretically calculated phase shifts at the silicon dioxide‐silicon interface based on known optical constants of silicon. Experiments comparing spectrophotometric oxide thickness determinations against VAMFO (variable‐angle monochromatic fringe observation) thickness determinations indicate an accuracy of ±25 Å and a precision of ±15 Å.