Graphitic carbon nitride (g-C3N4) has emerged as one of the most promising visible light active photocatalysts for NOx removal. Due to its low efficiency in bulk form though, many methods have been applied in order to improve the optical properties and photocatalytic activity. Among them, exfoliation via chemical and thermal treatment appears to be very effective and easy to perform. In this work, g-C3N4 was exfoliated by high-yield chemical and thermal treatment. A thorough comparative investigation was performed, revealing successful exfoliation of g-C3N4 and significantly enhanced visible light photocatalytic activity. Both methods led to high pore volume and specific surface area. It was established that chemical exfoliation resulted in wider band gap with more positive VB edge in comparison to thermal exfoliation. Furthermore, increased superoxide radical formation and reactivity of photogenerated electrons was demonstrated by EPR measurements in the case of chemical exfoliation. The chemically exfoliated g-C3N4 showed superior photocatalytic performance in NOx removal under visible light irradiation. This outcome was ascribed to the increased superoxide radical formation, the favorable band gap edges and porosity of the materials.