  The C--to--O ratio is a crucial determinant of the chemical properties of
planets. The recent observation of WASP 12b, a giant planet with a C/O value
larger than that estimated for its host star, poses a conundrum for
understanding the origin of this elemental ratio in any given planetary system.
In this paper, we propose a mechanism for enhancing the value of C/O in the
disk through the transport and distribution of volatiles. We construct a model
that computes the abundances of major C and O bearing volatiles under the
influence of gas drag, sublimation, vapor diffusion, condensation and
coagulation in a multi--iceline 1+1D protoplanetary disk. We find a gradual
depletion in water and carbon monoxide vapors inside the water's iceline with
carbon monoxide depleting slower than water. This effect increases the gaseous
C/O and decreases the C/H ratio in this region to values similar to those found
in WASP 12b's day side atmosphere. Giant planets whose envelopes were accreted
inside the water's iceline should then display C/O values larger than those of
their parent stars, making them members of the class of so-called ``carbon-rich
planets''.
