  We analyse the phenomenological implications of a particular class of
supersymmetric models with additional space-time dimensions below the
unification scale. Assuming the unification of the gauge couplings and using a
two-loop calculation below the scale of the additional space-time dimensions,
we show that the value of $\alpha_3(M_z)$ is further increased from the
two-loop Minimal Supersymmetric Standard Model prediction. We consider whether
decompactification threshold effects could bring $\alpha_3(M_z)$ into agreement
with experiment and discuss the associated level of fine tuning needed.
