The appropriate selection of controlled variables is one of the most important tasks in plantwide control. In this paper, we consider the selection of secondary temperature measurements for indirect composition control of distillation columns. The maximum gain rule (maximize minimum singular value of scaled gain matrix) is compared to the exact local method, and it is found that the gain rule should be used with care for ill-conditioned plants like distillation columns. We also consider the use of optimal combinations of measurements to further reduce the effect of disturbances and implementation errors.