The main purpose of this study was to examine and compare the performance of two extreme value distribution functions, the Gumbel and the Weibull CDFs, in modelling the distribution of the minimum and the maximum values of representative sets of tree diameter samples of Q. robur, B. pubescens, P. radiata, P. pinaster and P. sylvestris in northwest Spain. This is the first example of the application of the Gumbel CDF in forest modelling.