Loss on ignition (LOI) is a simple, inexpensive method widely used to estimate organic matter in the water column and sediment of marine and freshwater ecosystems. Suspended particulate matter in estuarine waters however often contains a large fraction of argillaceous minerals whose loss of structural water can introduce significant bias to the method. The accuracy of LOI, in comparison with elemental analysis, was evaluated in 105 water samples from the Marennes-Oléron Bay (French Atlantic coast). The suspended particulate matter analysed ranged from 7 to 108 mg l⁻¹, with a mean inorganic fraction of 81.3±s.d. 7.2%. LOI overestimated particulate organic matter (POM) concentrations by up to 300% (mean 98 ± s.d. 28%), and this overestimation was significantly related to particulate inorganic matter (PIM) concentrations. POM overestimation increased linearly with PIM, representing a mean 8.7±s.d. 2.0% of PIM. This value was compared with a theoretical correction factor based on the specific clay composition of PIM at the study site and on thermo-gravimetric analysis for standard clay. The clay fraction, which represents 90% of PIM in the Marennes-Oléron Sound, consists of 40% illite, 30% kaolinite and 30% montmorillonite. The similarity between the theoretical correction factor (9.3% of PIM) and the overestimation based on measurements (8.7%), suggests that a priori correction of POM data is possible. In fact, for three bodies of water along the French Atlantic coast, whose inorganic resuspended matter shows the same mineralogical composition, POM concentrations as estimated by LOI can be corrected by subtracting 9.3% of PIM mass.