  Using results on the mass gap in the sine-Gordon model combined with the
exact amplitudes in the bosonized representation of the Heisenberg spin-1/2
chain and one-loop renormalization group, we derive a quantitative expression
for the gap in a dimerized spin-1/2 chain. This expression is shown to be in
good agreement with recent numerical estimates when a marginally irrelevant
perturbation is taken into account.
