This study investigates the relationship between selected predictors of achievement in playing unrehearsed music (sight reading) and the changing complexity of sight reading tasks. The question under investigation is, how different variables gain or lose significance as sight reading stimuli become more difficult. Fifty-two piano major graduates and undergraduates took part in an experiment which consisted of five different levels of sight reading complexity. Predictor variables were divided into three categories: (i) general cognitive skills (e.g. working memory capacity); (ii) elementary cognitive skills (e.g. reaction time); and (iii) expertise-related skills (e.g. accumulated sight reading or inner hearing). Regression analyses indicate that when sight reading stimuli is easy, general pianistic expertise is sufficient to be able to excel. However, with increasing task difficulty, psychomotor speed (as indicated by trilling speed), speed of information processing, inner hearing and sight reading expertise become more important. When sight reading complexity reaches its highest level, sight reading expertise still remains important, but psychomotor speed becomes the dominant predictor. Results indicate (i) that psychomotor speed and speed of information processing have a "bottleneck function" and (ii) that there is a critical time window up to the age of 15 when sight reading expertise has to be acquired. It is concluded that with increasing task demands, sight reading ability is determined by both practice dependent skills and skills which are also assumed to be limited by innate abilities such as psychomotor movement speed. Thus we explain sight reading achievement as the result of specific combinations of different categories of skills which change with the demands of a task. (Contains 14 tables and 2 figures.)