Abstract: A method is described and claimed for the replenishment of deuterium or tritium concentration in a laser isotope separation process for the production of D.sub.2 O, or for the recovery of tritium from contaminated D.sub.2 O or H.sub.2 O. A working compound is selectively photodissociated by a laser beam and is replenished by contacting an exchange liquid in a countercurrent fashion in a first contacting column. The exchange liquid is replenished with isotope by in turn being contacted with a feed stream in a second contacting apparatus. This second contacting apparatus may be a gas/liquid or liquid/liquid contacting apparatus and the countercurrent flows therein may be about equal or unequal.