This study sought to establish the effect of sin activities on the financial performance of companies listed at the Nairobi Securities Exchange. The study adopted a descriptive research design with the population consisting of all 63 listed firms in the NSE as at December 2013. The sample of the study consisted of the 20 high performing NSE companies. At the time there are only two sin companies in this index listed. Furthermore this study grouped 18firms into the non-sin companies¿ category and another 2 firms (BAT ad EABL) into the sin companies group. Secondary data was gathered from NSE financial reports data base for analysis which was done using the Statistical Package for Social Sciences (SPSS version 19) to generate the descriptive statistics and also to generate the trends results and correlation findings. One sampled T-Test was used to check whether the mean performance of Sin companies differ from the mean performance of non-sin companies. Findings on the analysis of variance (ANOVA) indicate that the overall model was statistically significant as indicated by an F statistic of 2.943 and p value less than 0.0498.