A new method to depth profile N and O concentrations through sub-30 Angstrom silicon oxynitride films by time-of-flight secondary ion mass spectrometry (ToF-SIMS) is examined. Using secondary Si+ matrix ions to cationize neutral molecular and elemental species, quantitative N and O concentration (concn) data are obtained through sub-30 Angstrom films with N concns of 12 atom % or less. Concentration depth profiles through 10 Angstrom thick silicon oxide films are achieved. Isobaric mass interference with the Si2N+ peak at unit atomic mass resolution elevates the N background concn to 0.25 atom % at a 5 Angstrom depth. At higher mass resolution (m/Delta m = 5,500 full width at half-maximum), the N background is less than 0.1 atom % at 1 Angstrom, achieving a baseline level of 0.015 atom % after 10 Angstrom. Using a reference sample, variation in O and N depths and concentrations over several months is less than 2% relative standard deviation.