Leather furniture isn't as likely to get soaked as a leather jacket or purse, but the chances are good that a chair or sofa will sustain at least one spill during its lifetime. When that happens, you should use the same remediation strategy you would use for leather outerware and accessories. You don't want to allow the leather to dry out quickly, and you do want to replenish the oils the excess moisture draws out.
The Damage from Moisture
The microscopic fibers that hold a piece of leather together are lubricated by natural oils when the animal is alive, but these oils are washed away during the tanning process and replaced with others. When water penetrates, it partially dissolves these added oils and brings them to the surface, where they evaporate. As the skin dries out, the bonds between the fibers weaken -- eventually, the leather stiffens and cracks. Once this weakening of fibers occurs, it is irreversible. Rubbing oil into the leather makes it supple again, but this won't restore its strength.
First Aid for Wet Leather
The worst thing you can do to a leather chair or sofa that has gotten wet is to hasten the rate of evaporation by pointing a light or other heat source at it to dry it out. You can't stop evaporation, but you can slow it down by keeping the leather out of the sun and in a cool place. While the leather is still wet, rub in some pH-balanced leather conditioner. As the water slowly leaches to the surface, capillary action draws the oils in the leather conditioner into the pores. Be sure to use a neutral-pH product, because high-pH, caustic conditioners damage the fibers and weaken the leather.
Stay Off the Furniture
Before it dries out, wet leather becomes more supple and malleable than it was before, and it has an increased tendency to stretch. Once leather stretches, it won't regain its shape, so you must avoid using leather furniture while it's wet. If you have pets or children, you may have to take measures to discourage them from using the furniture, such as taping cardboard to the arms. If someone does use the furniture, and the leather stretches, avoid the temptation to try to shrink it back to its original shape by using heat. Instead, simply allow the leather to continue drying slowly.
Keep the Leather Ventilated
Don't try to inhibit evaporation by covering the furniture with plastic. This encourages the growth of mold, and once it starts, you may have a difficult time getting rid of it. Leather needs ventilation, and as long as you keep the ambient temperature low, mold shouldn't be a problem. If you do notice telltale signs of mold, such as a white film on the surface of the leather, you can protect your furniture by acting quickly. Rub the affected area with a 1-to-1 solution of vinegar and water, and then replace the surface oils by treating the leather with conditioner.