  Compressed S and Se are studied by x-ray diffraction with synchrotron
radiation up to 160 GPa. The S-IV phase is shown to have a body-centered
monoclinic structure and to be stable between 83 and 150 GPa on pressure
increase, where it transform to S-V with a primitive rhombohedral $\beta$-Po
structure. Observation of the modulation reflections in S-IV up to 135 GPa
shows that its crystal structure is incommensurately modulated, as recently
reported for Se and Te. The modulated body-centered monoclinic phase of Se,
Se-IV, is shown to transform to the $\beta$-Po phase Se-V at around 80 GPa.
Se-V transforms to a body-centered cubic phase at 140 GPa in accordance with
previous studies. Pressure dependence of the structural parameters of these
high-pressure phases in S and Se are discussed in relation to their
superconducting behaviour.
