The hydrodynamic and mass transfer performance of Karr reciprocating plate extraction columns with varying column diameters has been presented in order to examine how column performance changes with scale. An ideal liquid system of kerosene/tributyl phosphate-phenol-water was initially studied using a 50 mm diameter Karr column. Correlations were developed to predict the dispersed phase holdup, drop size distribution, and overall mass transfer coefficient over a range of operating conditions. This was followed by column performance studies using a phenolic alkaloid liquid system in Karr columns with diameters of 50, 100, and 300 mm. Overall results showed that there was no significant change in either the dispersed phase holdup or the mass transfer coefficient with column diameter. It was therefore concluded that overall column performance was independent of column diameter and the traditional Karr column scale-up equations were too conservative. This study also showed that factors such as droplet and plate coalescence, contamination of liquid systems, aging of column internals, and variation in physical properties can greatly influence the column's performance and need to be carefully considered when designing a Karr column.