Distillation columns with liquid or vapor sidestreams are utilized to accomplish a separation in one column which would otherwise require two or more columns and which will usually require higher utility usage. While sidestream. columns offer the economic incentives of reduced capital and operating costs, they also pose a more challenging control problem than ordinary distillation columns. Columns with a liquid or vapor sidestream drawoff have an additional degree of freedom and, as a result, have an additional dimension of coupling. Therefore, these columns are, in general, more difficult to control than standard distillation columns. This paper describes a concept for composition control of all product streams of a distillation column with a liquid sidestream. All product compositions are determined by on-line gas chromatographs and are characterized by high dead times. The resulting problem of sampled-data feedback control was investigated by using a dynamic simulator. Plant data, both steady-state and dynamic were used to validate steady-state and dynamic model of the column. The developed control concept and the diagnostic system for the plant operators were implemented in the DCS. Both elements show good performance and ensure product quality in operation.