I'm with PTSB too and have the same rate as you, and I've reduced the capital without any problem - however, I'm on variable rate. I'm not exactly sure about the interest only rate - can you be paying a tracker rate on an interest rate only mortgage? I'm not sure, but if you can maybe that's where the problem/issue is - and you'll end up paying off the interest first. If you don't get anywhere with whomever you contacted at your branch it might be worthwhile speaking to a manager. I'd find it hard to believe that a bank won't 'accept' money from someone willing to reduce the capital on a mortgage these days - but you never know, it's worth asking again I think. Let us know how you get on.