The role of content in formal operations tasks was explored in research that systematically varied the content of tasks and children's performance by developmental level and socioeconomic status (SES). Subjects were 110 Icelandic 12-year-olds who had entered a longitudinal study when they were 7 years old. Two extreme ability groups of children from Reykjavik primary schools were formed. Measures of formal operations employed were two tasks of variable isolation and measures of conservation of displacement volume. Variable isolation tasks were the pendulum problem and three content versions of the plant problem. Subjects were asked to isolate the operative variable and exclude the inoperative ones. Displacement of volume tasks were of two kinds: a formal Piagetian version and another version judged to be more experiential or familiar to subjects. Each content version was tested by means of three different forms of tasks, in which the shape or weight of the inserted object or the size of the container was varied. The experimental version involved a set of cartoon stories of social games. Three variations, which were similar to variations of the formal tasks, were presented. All tasks were administered, along with classification tasks and syllogisms, in a school setting in one session. Findings indicated that while formal operational thought differed by SES and gender, Piagetian tasks were biased for SES only. (RH)