The three-dimensional image stability space is introduced for evaluating the stability of color images in actual environments. Six typical conditions for storage or display of photographs are analyzed in terms of thermal ( X -axis), brightness ( Y -axis) and frequency ( Z -axis) factors, the last of which corresponds to the frequency of the storage or dirplay conditions in Japan. The three-dimensional expression ( X - Y - Z ) is helpful to survey the situation of natural aging of photographs and reveal that accelerated aging tests are appropriate for the evaluation of dark and light stability.