An analysis of interferometric phase delays from 15 years of Mark I and Mark III very long baseline interferometry (VLBI) experiments carried out with two radio telescopes in Westford, Massachusetts, ~1.24 km apart, yields weighted root-mean-square (WRMS) scatters about the mean locally horizontal coordinates of 1.0 and 2.0 mm in the north and east directions, respectively. The vertical coordinate scatter over this same duration is 3.2 mm. The measurements made during the last decade using the more accurate Mark III VLBI system yield WRMS scatters of 0.7, 0.8, and 2.3 mm, for the north, east, and vertical coordinates, respectively. Analysis of this latter data set using the less accurate, but more commonly used, group delay measurements yields WRMS scatters of 1.8, 2.5, and 2.8 mm for the north, east, and height components, respectively. The estimated rates of change of the coordinates of the baselines from the phase delay solution are -0.12¿0.04, -0.09¿0.07, and -0.26¿0.11 mm/yr for the 15-year analysis, and -0.11¿0.05, 0.08¿0.06, and -0.15¿0.15 mm/yr for the last decade. From these results, the maximum admissible rate of change of this baseline, either from geophysical causes or from telescope deformation, can be bounded to be less than 0.5 mm/yr (95% confidence interval) in all coordinate directions. We conclude from these studies that VLBI antennas of at least of the structural quality of the pair in Westford satisfy a necessary but not sufficient condition for being able to maintain a global reference system with submillimeter per year accuracy for intervals in excess of a decade. These data are also used to determine an error model for the VLBI group delay measurements, and, for this particular pair of telescopes, they indicate that the WRMS difference between group and phase delays is composed of a constant part (5.4 mm, for the most recent data) and a signal-to-noise ratio (SNR) term which is ~10% larger than that computed theoretically. ¿ American Geophysical Union 1992