This paper presents two tests of full-scale simply supported two-way concrete slabs in a high-temperature environment, based on the standard temperature–time curve (ISO 834). Details of the support conditions, the reinforcement arrangements and the positions of displacement transducers and thermocouple trees are given. Crack development, the temperature distribution, central vertical deflection and edge horizontal deflection of the concrete slabs were recorded during the tests. It was observed that, despite large deflection, neither slab collapsed; this was due to the significant influence of tensile membrane effects on maintaining the structural stability of the floor slabs under fire conditions. Based on the experimental results, an energy-based model to determine the limit load-carrying capacity of concrete slabs at elevated temperatures is verified and discussed. A comparison of the energy-based model results and the test results showed good agreement, while conventional yield theory provides conservative critical reinforcement temperatures in the slabs.