Hey, I am looking that the accfence1 test in MPICH and I think the tests is invalid. The test attempts to execute an MPI_Accumulate with different datatypes for the origin and destination. The problem is that the test violates MPI 3.0 § 11.3.4 p. 426 1-4: Each datatype argument must be a predefined datatype or a derived datatype, where all basic components are of the same predefined datatype. Both datatype arguments must be constructed from the same predefined datatype. The operation op applies to elements of that predefined type. I see no wording in the standard that says this restriction does not apply when the operator is MPI_REPLACE. The test is correct if the operation is changed from MPI_Accumulate to MPI_Put. If the test is correct then it might make sense to make it explicit in the standard that the datatype restriction does not apply to MPI_REPLACE. -Nathan