This is a study of the ex-dividend day. The study covers six years (2007-2012) and studies the shares included in the OMXS30 on the Stockholm stock exchange. OMXS30 is a share index of the 30 most actively traded stocks on the exchange. The study comprised a total of 145 observations. The purpose of this study is to investigate if the stock price on the ex-dividend day unfolds as the efficient market hypothesis teach or if there exists room for speculation, and thus earn a return higher than the market.The ex-dividend day effect has been studied previously, both on the Swedish market but also abroad. The results of previous researchers are different but they all have one thing in common, namely that there is a certain ex-dividend day effect.When calculating the ex-dividend day effect, the stock prices had to be adjusted for the normal return that occurs during the ex-dividend day. The normal return is not something that is universally known, but must be estimated by the author. In this study, it was estimated using the OMXS30 index movements relative to each company's beta. After that price drop ratio is calculated. The authors found that the share price on average fell by about 90 % of the dividend amount, with the possibility of an excess return of approximately 0.37 %. This was according to statistical tests significantly different from one, indicating that the ex-dividend day effect exists. By studying each year the authors found significant discrepancies between the years that cannot be explained, the authors themselves could conclude that the price drop ratio exists in symbiosis with the current economic situation.