Context : The abundances of \alpha -elements are a powerful diagnostic of the star formation history and chemical evolution of a galaxy .
Sulphur , being moderately volatile , can be reliably measured in the interstellar medium ( ISM ) of damped Ly- \alpha galaxies and extragalactic H ii regions .
Measurements in stars of different metallicity in our Galaxy can then be readily compared to the abundances in external galaxies .
Such a comparison is not possible for Si or Ca that suffer depletion onto dust in the ISM .
Furthermore , studying sulphur is interesting because it probes nucleosynthetic conditions that are very different from those of O or Mg .
In this context measurements in star clusters are a reliable tracers of the Galactic evolution of sulphur .

Aims : The aim of this paper is to determine sulphur abundances in several Galactic clusters that span a metallicity range -1.5 < { [ Fe / H ] } < 0.0 .

Methods : We use a standard abundance analysis , based on 1D model atmospheres in local thermodynamical equilibrium ( LTE ) and literature corrections for non-local thermodynamical equilibrium ( NLTE ) , as well as 3D corrections based on hydrodynamical model atmospheres , to derive sulphur abundances in a sample of stars in the globular cluster M 4 , and the open clusters Trumpler 5 , NGC 2477 , and NGC 5822 .

Results : We find \langle { A } \left ( { S } \right ) \rangle _ { NLTE } = 6.11 \pm 0.04 for M 4 , \langle { A } \left ( { S } \right ) \rangle _ { NLTE } = 7.17 \pm 0.02 for NGC 2477 , and \langle { A } \left ( { S } \right ) \rangle _ { NLTE } = 7.13 \pm 0.06 for NGC 5822 .
For the only star studied in Trumpler 5 we find A ( S ) _ { NLTE } = 6.43 \pm 0.03 and A ( S ) _ { LTE } = 6.94 \pm 0.05 .

Conclusions : Our measurements show that , by and large , the S abundances in Galactic clusters trace reliably those in field stars .
The only possible exception is Trumpler 5 , for which the NLTE sulphur abundance implies an [ S/Fe ] ratio lower by roughly 0.4 dex than observed in field stars of comparable metallicity , even though its LTE sulphur abundance is in line with abundances of field stars .
Moreover the LTE sulphur abundance is consistent only with the abundance of another \alpha -element , Mg , in the same star , while the low NLTE value is consistent with Si and Ca .
We believe that further investigation of departures from LTE is necessary , as well as observation of other S i lines in this star and in other stars of the same cluster , before one can conclude that the sulphur abundance in Trumpler 5 is indeed 0.4 dex lower than in field stars of comparable metallicity .
The S abundances in our sample of stars in clusters imply that the clusters are chemically homogeneous for S within 0.05 dex .