  The Space Interferometry Mission (SIM), with its launch date planned for
2005, has as its goal astrometry with ~ 1 micro-arcsecond accuracy for stars as
faint as 20th mag. If the SIM lives to expectations it can be used to measure
astrometric displacements in the light centroid caused by gravitational
microlensing in the events detected photometrically from the ground. The effect
is typically ~ 0.1 mas, i.e. two orders of magnitude larger than planned SIM's
accuracy. Therefore, it will be possible to determine the mass, the distance,
and the proper motion of almost any MACHO capable of inducing a photometric
microlensing event towards the galactic bulge or the Magellanic Clouds, even
though no light from the MACHO has to be detected.
  For strong microlensing events in which the source is photometrically
resolved, like the recent MACHO 95-30 event, SIM's astrometry combined with
accurate ground based photometry will allow the determination of the angular
stellar radii, and therefore the effective temperature of the source.
  The effective astrometric cross sections for gravitational lensing by nearby
high proper motion stars and brown dwarfs are ~ (1'')^2 and the effective time
scales are ~ 1 year. SIM will provide the only practical way to measure masses
of single nearby objects with ~ 1% accuracy. The times of lensing events can be
predicted years ahead of time.
