The effective design of chemical processes requires robust and efficient algorithms for single-stage vapor-liquid flash calculations. These flash calculations represent a critical link between thermophysical properties and process and equipment design. The "inside-out" formulation proposed by Boston and Britt (1978) has proved to be a work-horse algorithm in the in-house software used at Air Products and in commercial process simulators such as AspenPlus(TM). In this paper we describe enhancements that improve the efficiency of the Boston-Britt algorithm and extend it to new flash types. A new technique is proposed such that single-phase solutions (either in the vapor or liquid regions, or in the supercritical region) are detected reliably and efficiently. Further, the Boston-Britt algorithm is extended to new flash types, such as pressure-entropy and pressure-internal energy. Our in-house implementation of the proposed enhancements has been validated by application to a wide variety of chemical processes. In the present paper, the efficiency and power of the proposed enhancements are demonstrated by application to cryogenic design calculations. (C) 1998 Published by Elsevier Science Ltd. All rights reserved.