This paper deals with the effects of initiator concentration ([I]) on the styrene (STY) emulsion polymerisation stabilised by the mixed anionic/nonionic surfactants, sodium dodecyl sulfate (SDS)/nonylphenol polyethoxylate with an average of 40 ethylene oxides per molecule (NP-40). The reaction system stabilised only by SDS ([NP-40] = 0 wt%) results in an increase in the rate of polymerisation (R-p) with [I]. For the system stabilised only by NP-40 ([NP-40] = 100 wt%), R-p remains relatively constant when [I] increases. For the system with [NP-40] = 50 or 80 wt%, R-p first increases to a maximum and then decreases with increasing [I]. Furthermore, the total scrap, presumably caused by bridging flocculation and/or formation of complex between the ethylene oxide units of NP-40 and the particle surface SO4- end-groups derived from the persulfate initiator, first remains relatively constant and then increases rapidly to a maximum with increasing [I]. Beyond the maximum, the total scrap starts to decrease with increasing [I]. Smith-Ewart case II theory (i.e., n(s) = 0.6 and n(i) = 0.4 in the relationship N-p similar to[S](ns)[I](ni)) is only applicable to the system with [NP-40] = 0 wt%. At [I] = 1.38 x 10(-3) M, systems with [NP-40] = 0, 50 and 80 wt% result in comparable latex particle sizes (d(p)) and relatively monodisperse size distributions throughout the reaction. On the other hand, the system with [NP-40] = 100 wt% shows the largest d(p) and the broadest size distribution, which is attributed to the long particle nucleation period and/or limited flocculation.