The analysis of complex mixtures of chlorinated paraffins (CPs) with short (SCCPs, C(10)-C(13)) and medium (MCCPs, C(14)-C(17)) chain lengths can be disturbed by mass overlap, if low resolution mass spectrometry (LRMS) in the electron capture negative ionization mode is employed. This is caused by CP congeners with the same nominal mass, but with five carbon atoms more and two chlorine atoms less; for example C(11)H(17)(37)Cl(35)Cl(6) ( m/ z 395.9) and C(16)H(29)(35)Cl(5) ( m/ z 396.1). This can lead to an overestimation of congener group quantity and/or of total CP concentration. The magnitude of this interference was studied by evaluating the change after mixing a SCCP standard and a MCCP standard 1+1 (S+MCCP mixture) and comparing it to the single standards. A quantification of the less abundant C(16) and C(17) congeners present in the MCCP standard was not possible due to interference from the major C(11) and C(12) congeners in the SCCPs. Also, signals for SCCPs (C(10)-C(12)) with nine and ten chlorine atoms were mimicked by MCCPs (C(15)-C(17)) with seven and eight chlorine atoms (for instance C(10)H(12)Cl(10) by C(15)H(24)Cl(8)). A similar observation was made for signals from C(15)-C(17) CPs with four and five chlorine atoms resulting from SCCPs (C(10)-C(12)) with six and seven chlorine atoms (such as C(15)H(28)Cl(4) by C(10)H(16)Cl(6)) in the S+MCCP mixture. It could be shown that the quantification of the most abundant congeners (C(11)-C(14)) is not affected by any interference. The determination of C(10) and C(15) congeners is partly disturbed, but this can be detected by investigating isotope ratios, retention time ranges and the shapes of the CP signals. Also, lower chlorinated compounds forming [M+Cl](-) as the most abundant ion instead of [M-Cl](-) are especially sensitive to systematic errors caused by superposition of ions of different composition and the same nominal mass.