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Alkyl- and aryl-esters of phosphoric acid (both halogenated and non-halogenated) are mainly used as flame retardants (FRs), among other applications, in furniture and consumer products and they are collectively known as organophosphate flame retardants (OPFRs). The absorption, biotransformation or elimination of many of these chemicals in humans and their possible health effects are not yet well known. A major reason for the limited information is the nature of these compounds, which causes several technical difficulties in their isolation and sensitive determination. A novel analytical liquid chromatography tandem mass spectrometry (LC–MS/MS) method for the accurate and sensitive determination of four urinary OPFR metabolites: bis(1,3-dichloro-2-propyl) phosphate (BDCIPP), bis(2-chloroethyl) phosphate (BCEP), bis(1-chloro-2-propyl) phosphate (BCIPP), and diphenyl phosphate (DPhP), using mixed-mode solid phase extraction and isotope. For the first time all four analytes can be identified in one chromatographic run. An extensive investigation of method development parameters (enzymatic hydrolysis, matrix effects, process efficiency, sources of background interferences, linearity, accuracy, precision, stabilities and limits of detection and quantification) was performed in order to address previously reported method inconsistencies and select a process with the highest accuracy and sensitivity. Chromatographic separation was achieved on a Luna C 18 (2) (2.00 mm × 150 mm, 3 μm) with mobile phase 80:20 v/v water: MeOH and MeOH: water 95:5 v/v, both containing 1 mM tributylamine and 1 mM acetic acid. Limits of detection were 0.025 ng mL −1 for BDCIPP and BCIPP and 0.1 ng mL −1 for DPhP and BCEP. Absolute recoveries of all four analytes and their labeled compounds were in the range of 88–107%. The method was tested on 13 adult California urine samples. BCEP was detected at 0.4–15 ng mL −1 with a geometric mean (GM): 1.9 ng mL −1 ; BDCIPP at 0.5–7.3 ng mL −1 , (GM: 2.5 ng mL −1 ) and DPhP at