This article reports on the second language (L2) perception of contrasts among British English monophthongs. This study has two aims: 1) to explore the discriminability of contrasts in L2 British English monophthongs; and 2) to test the perceptual assimilation model-L2 (PAM-L2) towards the ability to discriminate British English contrasts. The contrasts considered were: /i?/-/[near-close near-front unrounded vowel]/, /ae/-[open-mid back unrounded vowel]/, /[ezh]?/-/[open-mid back unrounded vowel]/, /u?/-/[near-close back rounded vowel]/, /e/-/ae/, /[open-mid back unrounded vowel]/-/[open back rounded vowel]/, /[near-close back rounded vowel] /-/[open-mid back rounded vowel]?/, /[open back unrounded vowel]?/-/[open-mid back unrounded vowel]/, /[esh]?/-/[open-mid back rounded vowel]?/. Fifty-two native speakers of Thai who were learning English as a foreign language in Thailand participated in a forced-choice ABX discrimination task. The participants were divided between two groups -- those high-experienced and those low-experienced. The results evidence how both groups performed well on most contrasts (over 80% correct), except for /[open-mid back unrounded vowel]/-/[open back rounded vowel]/. Although the discriminability of the contrast /[open-mid back unrounded vowel]/-/[open back rounded vowel]/ was lower than with the other contrasts, the discrimination scores among both groups remained in a middle range (over 70%). No effect of L2 experience was found, thus suggesting that the abilities of both groups did not differ. The PAM-L2 was accurate in predicting that neither group of L2 Thai learners would have difficulty in discriminating the considered L2 sound contrasts. These results imply that the results gained from a perceptual assimilation task are useful in predicting the discriminability of L2 sound contrasts, as suggested by the PAM-L2.