This 4GB RAM is 100% compatible with the MSI H81M-P33. Help in selecting the right RAM: We offer you faster 1600MHz RAM for your system. Due to the backward compatibility of our RAM with 1333MHz, you can also mix it with existing 1333MHz memory. RAM with ECC provides higher security through error detection and correction. The ECC function is supported when all existing memory is equipped with ECC. You should use UDIMM ECC RAM if existing memory is already equipped with ECC. It is possible to use different RAM sizes in the slots. The Dual Channel Mode is also supported through Intel Flex Memory Technology. When using different RAM capacities, you need to ensure that the memory in memory channel A is larger than the memory in memory channel B.