In this study, the synthesis of a mesoporous crystalline covalent organic framework (COF) based on melamine and 1,4-dibromo butane as COF-1,4 and its use as a template for in situ synthesis of conductive polymers such as poly(Aniline) (PANi) and poly(Pyrrole) (PPy) within the pores were reported. The synthesized COF-1,4/conductive polymer semi-Interpenetrating Network (semi-IPN) composites were characterized via FT-IR spectroscopy and thermogravimetric analyzer (TGA), and the conductivities of the prepared composites were determined with an electrometer. It was found that upon the in situ synthesis of the conductive polymers such as PANi and PPy within COF-1,4, the conductivity of bare COF-1,4 was increased 3million-fold for COF-1,4/PANi, and 500 thousand-fold for COF-1,4/PPy, respectively. Furthermore, the potential sensor applications of these COF-1,4/conductive polymer semi-IPN composites were investigated against HCl and NH3, and methyl orange (MO), and methylene blue (MB) dyes in aqueous solutions. The sensor studies revealed that the conductivity of bare COF-1,4 increased 20 and 7K-folds upon 15min exposure to HCl and NH3 gases, respectively. Interestingly, an eightfold decrease in the conductivity of COF-1,4/PANi was observed upon 15min exposure to NH3 gas vapor at ambient conditions. Also, the conductivities of prepared COF-1,4 and its conductive polymer composites changed after treatment with MO and MB dyes suggesting other potential sensor applications of these porous materials.