The paper deals with the effect of operating conditions on small-scale randomly-packed column separation performance during distillation of high-purity liquefied gases under elevated pressure (up to 25 x 10(5) Pa). Examination of mass-transfer performance under elevated pressure at a fixed distance from flood has been made for high-performance spiral-prismatic packings of 4 mm and 2 mm nominal size using silane- and ammonia-based test systems. Impact of simultaneously changing pressure, temperature and flow rates (loading kept at 80% of flooding) on the column separation efficiency has been investigated experimentally and by numerical simulation: variations of steady-state column separation factor and the height equivalent to a theoretical plate (HETP) at total reflux conditions are presented and discussed. Observations made demonstrate that overall column separation efficiency shows an extreme behavior - maximum separation effect is achieved under certain temperature (pressure) depending on the variation of the HETP and vapor-liquid equilibrium of the matrix-impurity system. Dependencies obtained are similar both for ammonia- and for silane-based systems and are in agreement with those predicted by analytical method based on empirical correlative approach found in the literature. (C) 2014 Elsevier B.V. All rights reserved.