[en] The interaction between hydrogen and defects in iron has been studied by the positron lifetime technique. Hydrogen binding to voids is observed to reduce their positron trapping rate and for very small voids also the positron lifetime is reduced. The H binding energy in voids is determined: E/sub B/ = 0.77 eV, and it is argued that H chemisorbs to void surfaces. The thermal H-release stage is more than 200 K wide and extends up to 500 K indicating the existence of a potential barrier at the sample surface obstructing the H-release