Can't use Superscript in tabular!

[vsharma]

HI
I am trying to use the chemical formula SO4=/COO= with the 4 as subscript 
and the equal sign (I tried -2 as well) as superscript in a tabular cell 
but i get errors.
 Is there some way to do it.
I have tried Math mode and using the super and subscript functions from 
Insert - special character and i get the same errors
The formatting outlside the tabular cell works fine when i see it in DVi
thanks
vivek