  For a given spatial distribution of the lenses and distribution of the
transverse velocity of the lens relative to the line-of-sight, a probability
distribution for the lens mass for a single observed event is derived. In
addition, similar probability distributions are derived for the Einstein radius
and the separation of the lens objects and their rotation period for a binary
lens. These probability distributions are distinct from the distributions for
the lens population, as investigated e.g. by the mass moment method of De
Rujula, Jetzer, and Masso (1991). However, it is shown that the expectation
value for the mass from the probability distribution coincides with the value
from the mass moment method applied to a single observed event. The special
cases of a Maxwellian velocity distribution and of a constant velocity are
discussed in detail. For a rudimentary model of the Galactic halo, the
probability distributions are shown and the relations between the expectation
values of the physical quantities and the event timescale are given. For this
model, it is shown that within a 95.4%-interval around the expectation value,
the mass varies by a factor of 800.
