The cement annulus between a wellbore and a casing string should provide reliable zonal isolation throughout the life of a well. However, zonal isolation may be negatively affected by several factors which can compromise cement integrity and lead to the development of channels that provide migration paths for hydrocarbons. This paper is dedicated to monitoring the unwanted presence and migration of hydrocarbons in cemented annuli using a sophisticated fiber optic sensing system. The system is comprised of a distributed temperature and strain sensing (DTSS) data acquisition unit and a newly designed fiber optic cable that is capable of detecting hydrocarbons in cemented annuli. The DTSS system is based on Brillouin and Rayleigh backscattering mechanisms, which are sensitive to both strain and temperature changes. The cable consists of a single-mode optical fiber packaged with a hydrocarbon-sensitive polymer. This combination was tested extensively and successfully for its ability to identify cement integrity issues through detection of hydrocarbons in the cement. The presence of hydrocarbons causes the polymer around the fiber to swell, leading to changes in strain distribution on the fiber. These strain variations were detected using the DTSS monitoring system. The observed strain variations were found to be dependent on the type of fluids, which indicates that the proposed system has selective sensitivity to certain hydrocarbons. For example, synthetic-based mud had little effect on strain, while kerosene showed a significant response. In conclusion, the new fiber optic sensing system can be beneficial in verifying zonal isolation by detecting and monitoring any unwanted hydrocarbon migration in cemented annuli, identifying zones from which hydrocarbons are originating, and providing other essential information to identify the need for well intervention such as remedial cementing.