This paper reports the factors that three Year 11 mathematics students identified as influencing their choice to use a Computer Algebra System (CAS) for solving selected problems, some of which were expected to be outside their pen-and-paper (p&p) range. In individual interviews, the students were presented with eleven algebra problems and were asked to indicate the items that they would choose CAS for, and to provide a rationale for their choices. Factors that influenced students' choices included their perception of the amount of p&p working required, their p&p facility, the complexity of the problem and the presence of words in the problem statements which were CAS commands (for example, solve).