  We report a theoretical study of sulphur chemistry, as applied to hot cores,
where S-bearing molecular ratios have been previously proposed and used as
chemical clocks. As in previous models, we follow the S-bearing molecular
composition after the injection of grain mantle components into the gas phase.
For this study, we developed a time-dependent chemical model with up-to-date
reaction rate coefficients. We ran several cases, using different realistic
chemical compositions for the grain mantles and for the gas prior to mantle
evaporation. The modeling shows that S-bearing molecular ratios depend very
critically on the gas temperature and density, the abundance of atomic oxygen,
and, most importantly, on the form of sulphur injected in the gas phase, which
is very poorly known. Consequently, ratios of S-bearing molecules cannot be
easily used as chemical clocks. However, detailed observations and careful
modeling of both physical and chemical structure can give hints on the source
age and constrain the mantle composition (i.e. the form of sulphur in cold
molecular clouds) and, thus, help to solve the mystery of the sulphur
depletion. We analyse in detail the cases of Orion and IRAS16293-2422. The
comparison of the available observations with our model suggests that the
majority of sulphur released from the mantles is mainly in, or soon converted
into, atomic form.
