Commercial-off-the-shelf (COTS) hardware and software is being evaluated and/or used in an increasing range of mission-critical distributed real-time and embedded (DRE) systems. Due to substantial R&D investment over the past decade, COTS middleware has recently matured to the point where it is no longer the dominant factor in the overhead, non-determinism, and priority inversion incurred by DRE systems. As a result, the focus has shifted to the COTS operating systems and networks, which are once again responsible for the majority of end-to-end latency and jitter.This paper compares and evaluates the suitability of popular COTS operating systems for real-time COTS middleware, such as Real-time CORBA. We examine real-time operating systems (VxWorks and QNX), general-purpose operating systems with real-time thread scheduling classes (Windows NT, Windows 2K, and Linux), and a hybrid real-time/general-purpose operating system (Linux/RT). While holding the hardware and ORB constant, we vary these operating systems systematically to measure platform-specific variations in context switch overhead, throughput of the ORB in terms of two-way operations per-second and memory footprint of the ORB libraries. We also measure how the latency and jitter of high-priority operations are affected as the number of low-priority operations increase.Our results indicate that real-time operating systems remain the platforms of choice to deliver predictable, efficient, and scalable performance for DRE middleware and applications. However, the emergence of hybrid general-purpose/real-time operating systems, such as Linux/RT, are a promising direction for future DRE systems. Although Linux/RT is not yet as deterministic as traditional real-time operating systems, such as QNX and VxWorks, it does provide more predictable and scalable behavior compared to mainstream operating systems, such as Windows NT/2K. Since traditional real-time operating systems tend to be expensive and tedious to configure/program, the maturation of Linux/RT will be a welcome advance for DRE system developers.