What would be causing the water from my washing machine to back-up into the kitchen sink after the washer has gone through the spin & drain cycle?

Can you please tell us where the washer is relative to the sink (i.e. is it upstairs, on the same level, or below)

Has this always happened or is this a new occurence?

Does it happen when you have a small load in or just when you have a large load in the washer?

I believe it is probably a partial blockage in the line after it connects to the sink drain line, but it could also be a vent problem... any additional information you provide would be very helpful.

This should help some with a diagnosis.
It's a fairly new occurence, and the water is coming higher up in the sink lately. The washer is located at the same level as the kitchen in the adjoining laundry room. And we only do large loads.

I would say that you have a partial blockage after the sink and laundry join meet. You may want to "blow out" the drain lines or have the lines routed.

The volume of water that a washer dumps into the drain lines can easily overwhelm an improperly sized drain. If the drain line is partially clogged it will make it even harder for the water to take the proper course and it will back flow up the "path of least resistence"... in this case it is your sink drain.

You can ask around and see if anyone you know has a "Snake" (sold at most large hardware stores)... or call a plumbing company.

I think you should be able to blow out the drain from the sink side since it seems to be a clog after the connection to the laundry drain.

Finally, check that your washer has a lint filter... this may be the source of your problem.

Thanks so much Animater, I'm gonna dig up the manual from the washer and see if there is a lint filter (I have no idea if there is one!) Luckily my neighbor has a snake.