In studying the interactions of organic compounds with dissolved organic matter (DOM), a conceptual question is whether the compound distribution between free and DOM-bound states depends on the compound concentration. It is important to know whether the binding isotherm is linear and may be described by a constant distribution coefficient. This knowledge is significant both for mechanistic understanding of organic compound-DOM binding (i.e. extent of similarity of compound-DOM interactions to a "partitioning" between water and "bulky" DOM) and for a capability to predict the DOM effect on pollutant fate. We address this question by examining pyrene fluorescence in effluent DOM solutions originated from the Greater Haifa wastewater reclamation complex. Fluorescence of pyrene-DOM solutions was recorded against pyrene concentration in effluent DOM samples from Haifa plant and the Qishon reservoir (before and after biodegradation of the more labile DOM fraction). Change of the solution fluorescence upon pyrene-DOM mixing (relatively to the separated reactants) evidenced for strong pyrene interactions with DOM. Pyrene binding capacity of DOM was estimated to be about 0.1% w/w. Hence, non-linear pyrene-DOM binding isotherm is characterized by a pyrene distribution coefficient varying, at least, by 4-7 times depending on pyrene concentration. Examination of the pyrene-DOM solution fluorescence vs pyrene concentration is more appropriate test for binding non-linearity as compared with a regular Stern-Volmer plot at one selected pyrene concentration. Findings suggest that a "partitioning" mechanism is hardly relevant for studied pyrene-DOM interactions. It is concluded that the Stern-Volmer plot is helpful for separating different quenching mechanisms and examining the incomplete quenching effect when pyrene-DOM solution fluorescence is linear function of the pyrene concentration. However, when solution fluorescence is not linear function of the pyrene concentration, variable extent of occupation of binding sites at DOM makes the fluorescence data analysis much more complicated.