Hi Yve and welcome to GoY. Before we can help you we need to know more. Is you soil acid or alkaline? Is the rhododendron in a pot or in the ground? Have you had a long spell of dry weather? Is the shrub in semi-shade? Once we have the answers to those questions we are in a better position to help.

It would also be useful if you put the county you live in on your profile... growing plants in the SE of England is a whole different ball game to growing them in the NE of Scotland.

Mineral deficiency? Try a little Sequestrene - once a year usually in spring - should help.

Thanks for the replies.

I live in the S.East (kent) we have had a long dry spell and it is sitting in a pot in alkaline compost in semi shade. It`s also in flower and has just been repotted (3 weeks ago)

Alkaline compost is probably the trouble, because in this compost it is unable to take up iron, and this will be why the leaves are yellowing. It needs acid compost and an urgent feed with ericaceous feed, sequestered iron or iron sulphate crystals. It shouldn't have been repotted when coming into flower, but you are going to have to do it again if you don't want to lose it. Semi shade is the right position.

You definitely need to get into ericaceous compost a.s.a.p. or it isn't going to survive... Though I have never heard of 'alkaline' compost!

Have you been watering in your drought conditions?