Measurement of inherent optical properties (IOPs) of seawater using an AC-9 dual-beam spectrophotometer has become routine on many oceanographic cruises. AC-9 data (a(AC9)(lambda), b(AC9)(lambda) and c(AC9)(lambda)) are frequently used in radiance transfer calculations of water leaving radiances, for use in remote sensing applications. There are at present, however, no generally accepted protocols for the inversion of in situ IOP spectra to obtain estimates of water composition. A model is presented to partition in situ IOP spectra between particulate and dissolved shelf seawater constituents using constituent-specific optical cross-sections. These partitioned IOP spectra are subsequently inverted, yielding estimates of optically significant constituent concentrations. The inversion of IOPs measured in situ enables the calculation of spatial and temporal variability in shelf seawater composition at greatly increased resolution when compared with traditional sample collection and analysis. On moorings, towed bodies and ferry-box systems, this data is greatly needed for validating remote sensing products and to provide information on shelf sea processes.