We consider the gauge neutral matter in the low--energy effective action for
string theory compactification on a \cym\ with $(2,2)$ world--sheet
supersymmetry. At the classical level these states (the \sing's of $E_6$)
correspond to the cohomology group $H^1(\M,{\rm End}\>T)$. We examine the first
order contribution of instantons to the mass matrix of these particles. In
principle, these corrections depend on the \K\ parameters $t_i$ through factors
of the form $e^{2\p i t_i}$ and also depend on the complex structure
parameters. For simplicity we consider in greatest detail the quintic threefold
$\cp4[5]$. It follows on general grounds that the total mass is often, and
perhaps always, zero. The contribution of individual instantons is however
nonzero and the contribution of a given instanton may develop poles associated
with instantons coalescing for certain values of the complex structure. This
can happen when the underlying \cym\ is smooth. Hence these poles must cancel
between the coalescing instantons in order that the superpotential be finite.
We examine also the \Y\ couplings involving neutral matter \ysing\ and neutral
and charged fields \ymix, which have been little investigated even though they
are of phenomenological interest. We study the general conditions under which
these couplings vanish classically. We also calculate the first--order
world--sheet instanton correction to these couplings and argue that these also
vanish.
