Monte-Carlo simulations are used to calculate the primary yield of hydrogen atoms (GH.) Of water radiolysis versus linear energy transfer (LET) up to similar to 100 keV/mum, at 25 degreesC and 300 degreesC. The G(H). values calculated at 25 degreesC using protons show a maximum near 6.5 keV/mum before decreasing steeply at higher LET. At 300 degreesC, G(H) remains constant below similar to7 keV/mum, while at higher LET it decreases faster than do its corresponding 25 degreesC values. For different radiation types (protons,H-2(4), He-4(2+), Li-7(3+), and C-12(6+)) Of equal LET > 10 keV/mum, G(H). increases as the incident ion velocity increases. The results compare generally well with experiment.