This work studies the difficulties children have solving two-step word problems of a given structure (e.g., 7 lollypops/2 chocolate bars X 6 chocolate bars). Thirty-six children between the ages of 9 and 14 years were individually observed solving problems of the above type. Other tasks, repetition of the problem and model recognition, were used to probe children's representation of the problem. Analysis of children's errors on these tasks show that children's difficulties with this type of problems stem from different sources. The major ones were: understanding relationships such as the one contained in the statement "7 lollypops for every 2 chocolate bars"; given a ratio finding two new quantities that have the same ratio; and using two steps to solve the problems. These difficulties are not common to all word problems. Thus, it is important to have a clear description of the types of problems that children are asked to solve in order to uncover the particular sources of difficulty. (Author)