A process for extracting metal values especially zinc values from aqueous solutions of metal salts which comprises contacting the aqueous solution with an organic phase comprising a compound of the formula:
wherein each of R¹, R², R³ and R⁴, independently, represents an optionally substituted hydrocarbyl or hydrocarbyloxy group or R¹ and R² together with the attached phosphorus atom and/or R³ and R⁴ together with the attached phosphorus atom form a 5- to 8-membered heterocyclic ring.