Well, this is not the most elegant of solutions, but I think that it works. I basically took the guts of the two VarCov functions and added them together. It seems to work OK on my machine. My guess as to why it wasn't working before is that the VarCov functions are returning an array, but when your AddMatrix function calls the VarCov function, it is feeding it a range. There are some articles out there on using arrays in formulas, but the workaround below may work for you. Best of luck,

Ben

Function AddMatrix(rng As Range) As Variant

Dim i As Integer
Dim j As Integer
Dim colnum As Integer
Dim matrix() As Double

colnum = rng.Columns.Count
ReDim matrix(colnum - 1, colnum - 1)

For i = 1 To colnum
For j = 1 To colnum
matrix(i - 1, j - 1) = Application.WorksheetFunction.Covar(rng.Columns(i) , rng.Columns(j)) + _
Application.WorksheetFunction.Covar(rng.Columns(i) , rng.Columns(j))
Next j
Next i

AddMatrix = matrix

End Function