The study aiming at the evaluation of the variables, which result in poor condition of some documents, but excellent of others shows, that the most important factors, which are influencing the degradation of historical documents are the width of the ink line, which is correlated with the absolute amount of ink applied to the paper, pH of the area containing the ink and paper grammature. The regression model employed is able to describe more than 50% of the total variance in the L* values, which indicates the extent of degradation caused by the ink corrosion. Additional variance may be partly ascribed to factors, which we were not able to evaluate, but which have significant effect on the paper degradation, most notably environmental influences on the documents during the centuries of their storage. Based on the result of the study, it is now possible to predict the stability of historical documents containing iron gall ink by determining the three critical factors- grammature of paper, width of ink lines and pH of the paper containing the ink. The result is therefore useful for evaluation of a condition of the collection written with iron gall ink and thus allows for preservation planning and best use of resources.