This paper presents an experimental investigation into possible variations of mechanical properties of structural steel and concrete in composite cellular beams exposed to ISO 834 fire and cooled down phases. Four full-scale fire tests were performed on protected and unprotected beams under assumed service loads. Tensile stress-strain behavior of steel coupons taken from the beams and compressive strengths of concrete cores taken from the reinforced concrete slabs are studied. Material coupon tests for steel are carried out as per TS EN ISO 6892-1. As for the concrete, compression tests were conducted. Coupon test results reveal that, after fire testing, a maximum reduction ratio of 65% in ultimate strain is obtained for the unprotected beam samples. This indicates that the reductions in the mechanical properties of steel in the protected beams are much less when compared to those of the unprotected beams. It is also found that the maximum increase in post-fire strength/pre-fire strength ratios for concrete is 11% for the unprotected beam, while a 20 % decrease is recorded for water based protected cellular beam. For the protected specimens, the RC slabs were exposed to higher temperatures, and the compressive strength of concrete after testing was lower than that of the unprotected beam slabs.