The present study examined the effects of Classwide Peer Tutoring (CWPT) under two conditions--with reinforcement (CWPT+ R) and without reinforcement (CWPT-- R)--on 77 students' spelling performance and intrinsic interest in lower secondary school Integrated Science. The students displayed educational attainments in the average range. Both CWPT approaches led to significant improvements in spelling test performance. Both groups also evaluated the programme positively. However, the CWPT+ R group made significantly greater learning gains than did the CWPT-- R students. These appeared to be related to higher levels of academic responding. Despite their impressive spelling gains, students in the CWPT+ R group displayed significantly lower intrinsic interest in Integrated Science after involvement in CWPT. However, great caution should be exercised in interpreting this last result, as statistically significant effects werenot found when group differences at post-test were examined by way of t-test and analysis of covariance methods. In summary, the data from this study suggest: (a) that the CWPT method of instruction is useful for helping lower secondary school students, (b) that reinforcers may increase learning gains by encouraging higher rates of responding, but (c) that there is an indication, subject to caution, that these reinforcers may possibly depress intrinsic interest in an academic activity (science in the case of this study).[Copyright of Educational Psychology is the property of Routledge. Full article may be available at the publisher's website: http://dx.doi.org/10.1080/0144341990190206]