The grid in Guam works at 110 V and 60 Hz; the recorded sockets are Types A and B. 110 V instead of 230 V: check the rating plate before you plug in a hair dryer. An adapter does not change voltage. 60 Hz rather than 50 Hz matters only for motors and timers, not for power supplies.
The same socket system is used in American Samoa, the Marshall Islands, Micronesia and Palau, so one adapter covers all of them. Travelling further to the Cook Islands, New Zealand and the Solomon Islands means a different adapter. 5 of the 18 destinations in Oceania share the plug types of Guam.
Protective earth
Grounded and ungrounded sockets may occur
Earthed sockets (Type B) and unearthed ones (Type A) exist side by side. For metal-bodied appliances the protective-earth path is what matters.
Check the rating plate: 110 V suits “100–240 V” supplies, not single-voltage appliances. The 60 Hz supply matters for motors and mains clocks, not for chargers. Check your own device →
Guam supplies 110 V. Dual-voltage chargers are fine; 230 V-only appliances need a converter.
An adapter changes the plug shape only, not voltage or frequency. Guam supplies 110 V at 60 Hz – check the rating plate on your device. How to read a rating plate →
If a socket gets hot, crackles or smells burnt, unplug the device straight away.