The aim of this paper is to analyse the impact of quality certification (ISO 9000) on firms market value as well as its influence on volatility. Additionally, several factors are examined in order to find those which determine the results. The sample used contains all the firms that, while trading on the Spanish stock market, have ever obtained a quality certification based on ISO 9000 norms between 1993 and 1999. To estimate the stock market reaction we have estimated the mean abnormal change in the stock prices of the firms obtaining this certification. In estimating the variation in volatility we have used four tests, two parametric, one non-parametric and a proposal of a semi-parametric one. The results arrived at are significant since there seems to be a positive reaction to quality certification as well as an increase in volatility.

Key words: quality, ISO 9000 certification, returns, volatility.
JEL classification: M13, M31.