Abstract: The invention concerns an aqueous delayed-gelation solution and a method of selectively placing a delayed-gelation solution in a hydrocarbon well. According to the invention, the aqueous delayed-gelation solution a dissolved metal salt which, in use, hydrolyses to form a gel, wherein the solution contains suspended inorganic particles and the method comprises the steps of providing the aqueous delayed-gelation solution, and—injecting the solution into the hydrocarbon well, whereby the suspended inorganic particles reduce or block the flow of the delayed-gelation solution to relatively low permeability formation zones thereby selectively placing the solution in a relatively high permeability formation zone. Applications of the invention relate to the treatment of hydrocarbon wells.