We explore the use of two spectral measures calculated in ERB space for differentiating between the frication noise of sibilants in Mandarin Chinese and Korean. The peak frequency (peakERB) of the spectral representation was used to capture differences in front cavity size and a dif-fuseness index (DI) was used to capture the bandwidth of the peak. In both [a] and a high vowel context, the peakERB measure differentiated between Mandarin [s], [c], and [∫], and also between Korean [sh] and [s*] which, although considered to be articulated at the same place, differ in front cavity size due to the tighter lingual constriction of [s*]. The DI measure helped further differentiate the fricatives, with Mandarin [C] having a broader peak (higher DI) than [s] or [∫], and Korean [sha] having a broader peak than [s*a]. When applied to the L2 Korean productions of L1 Mandarin speakers, we found evidence for both Korean fricatives assimilating to Mandarin [s] before [a], and to Mandarin [c] before [i].