The influence of hydrogen content on the mechanical properties and size of dynamically recrystallized grains in commercially pure (CP) titanium and Ti-5Al-2.5Sn alloy was investigated. The alloys with hydrogen contents from 0.1 to 5.2 at.% were deformed in the cc-field at temperatures of 650degrees, 750degreesN with initial strain rates of 5 x 10(-4) s(-1). A decrease of the deformation temperature leads to a reduction in grain size and to a stress increase for all compositions. This is in good agreement with the well known relation between the recrystallized grain size (d) and the steady flow stress sigma(s)=kd(-n). At a given test temperature the steady state flow stress is four times lower and the grain size is about ten times greater in CP titanium in comparison with the Ti-5Al-2.5Sn alloy. Hydrogen alloying of the T5Al-2.5Sn alloy does not lead to a noticeable change in a. and d. However, an increase in hydrogen content from 0.1 to 5.2 at.% in CP titanium leads not only to a decrease in grain size by a factor of 2 but also to a decrease in flow stress (about 28%). This result is not in agreement with the above relation. This unusual behaviour may be due to two reasons: the influence of hydrogen on grain growth and the hydrogen effect on dynamic strain ageing. Both these effects are stronger in CP titanium.