MSI H81M-P33 Series User Manual page 15

PCI_E1~2: PCIe Expansion Slots
The PCIe slot supports the PCIe interface expansion card.
Important
When adding or removing expansion cards, always turn off the power supply and
unplug the power supply power cable from the power outlet. Read the expansion
card's documentation to check for any necessary additional hardware or software
changes.
PCIe 2.0 x1 Slot
25
PCIe 3.0 x16 Slot