CMOS MAX Vss?

[cyborg0 at GlobalEyes.net]

Me and some of my workmates have a problem right now.. 
We are trying to find out the max Vss of MOST cmos gate chips.
I have heard 5V, 15V, 18V, and I have read somewhere that it was only
capable of 5V, but, with so many dissenting views it made me wonder,
since I, too, have heard that CMOS is capable of a higher Vss from
various other sources...
anyone with answers, let me know..
rob