An apparatus and method for outbound I/O write-combining are described. In one embodiment the method includes the detection of a write transaction request directed to an I/O device. Once detected, it is determined whether an address associated with the detected write transaction falls within a predetermined write-combinable range of the memory mapped I/O space assigned to the I/O device. When the transaction address falls within a write-combinable range, data associated with the detected write transaction is stored within a buffer corresponding to the transaction address. Accordingly, one embodiment of the present invention provides write-combining for data written to an I/O port associated with a memory mapped I/O address space.