Hi Rob,
do you mean the = IsFormula as supplied by Bob Phillips? Or the IF
Worksheet Function.
I'm assuming you meant the IsFormula function which you should be using
with conditional formatting.

I pasted Bob's Function into a blank standard module then applied
conditional formatting to some cells using "=IsFormula(whatever the
address is of the cell with the Cond Format)" without the speech marks
in the "Formula Is" box of the Cond Formatting and it worked OK, cells
with a formula were formatted accordingly

Sorry about the delay, I must have just been leaving for work when you
replied.

Ken Johnson