Can someone point me to the reason why, in the words ประโยค and ประโยชน์ the second syllable seems to follow the tone rule of the first syllable?

These appear to be ข้อยกเว้น (exceptions to the rule) which other words like ประเทศ and กระทืบ follow.

If there are others which follow this exception, i'd also like to know.

Thanks to David for the PM, which made me realise that i've asked the same question some time ago about สำเร็จ. I guess i didn't store that information away in an accessible place.

For anyone who's interested, here are a couple of links which help with the understanding of my question. http://www.thai-language.com/ref/cluster-tone

http://www.thai-language.com/id/830222

Edited April 20, 201115 yr by bhoydy