The structure transition Pt(100) 1 × 1 → hex is observed by means of LEED intensities. The clean metastable 1 × 1 phase is prepared by intermediate NO adsorption. The transition to the hexagonal surface structure, which is an activated process, is initiated by raising the temperature. Integral intensities for both integer order spots as well as for superstructure spots are taken as function of time for constant elevated temperatures and as function of temperature for continuous temperature increase. The development of spot profiles is recorded as well. It turns out that the spot half width of integer order spots remains constant during the transition. Intermediate stages during the structure change show considerable background which is concentrated to streaks and indicates structural disorder. The superstructure spots grow up from the streaks giving first a 1 × 5 pattern from which the final structure results by spot splitting in the very last stage of the transition. Their intensity increase is delayed with respect to the decrease of integer order spots. They occur already with a comparably narrow half width, which remains constant in a first part of the transition. In the second part, especially during the final splitting, a further reduction is observed. The results are qualitatively discussed to give a geometrical model of the reconstruction based on the shift of atomic rows. Quantitative evaluation of the intensity-time dependences results in the activation energy of the transition, ΔE≈1.1 eV. From the temperature dependent measurements the surface Debye temperatures of the 1 × 1 and reconstructed phases are deduced to be roughly equal.