Alkylphenolpolyethoxylates (APEOs) have been widely used as nonionic surfactants in various industrial and commercial products. There are two different degradation pathways of APEO. One is the gradual degradation of AP(n)EO to AP(n-1)EO, to subsequent degradation products, and finally to alkylphenol (AP). The other is through the formation of alkylphenol ethoxy acetic acid (APEC). We examined the quantitation method for these substances related to APEO [NP(1-15)EO, OP(1-10)EO, NP(1-10)EC] by high-performance liquid chromatography/mass spectrometry. Method detection limits (MDL), calculated from the replicate analyses of spiked deionized water, were 0.0018 [NP(4)EO]-0.0095[NP(8)EO] (μg·l^-1), 0.0048[OP(5)EO]-0.0090[OP(7)EO] (μg·l^-1), 0.0038[NP(8)EC]-0.0072[NP(5)EC] (μg·l^-1). The quantitation method that we developed has sufficiently low detection limits and can be applied to the influent and final effluent of a sewage treatment plant.