Currency in Hon Kong & Macau? May I use China Yen at both places?

Most places in HK allows HK dollars only. You'd better prepare enough in advance in order to bring you a lot of convenience.

Macao allows MOP$, HK dollars and RMB. But the conversion ratio is 1:1 by many sellers and in many stores. So it's not an economic way for you to use other currency except MOP$. You need also prepare enough MOP$.