The wife will be travelling from the UK to thailand for a 28day holiday in the 2nd week of holiday she will be visiting Malaysia fro 3 days and then back to thailand and a week later back to Uk. She has a Thai and a British passport which one should she use

If she exited the Thai kingdom with the Thai Passport she should use that to re enter the kingdom.... she should have a white coupon of the immigration stapled to the passport.....when she leaves Thailand she can use the Thai passport and use British to get into Malaysia.... You entry a country with one passport use the same one to exit...

Cliobkk, perfectly right. We are a family of 4 and have 11 passports. European, Australian and Thai.

In fact to enter and to exit Thailand, please use the Thai passport, for any other destination use your british passport. Easy.

Yes ^^^

DO NOTE however that she won't be able to swap passports if travelling via a land border, each country expects to see the exit stamp of the other. As she's only going for a few days she can use her Thai PP to enter Malaysia and get a 30 day (IIRC) visa exempt stamp.

Thanks alot no more info needed