This contribution deals with the determination of the reference temperature of A533 Cl.1 steel using miniaturized specimens. The dimensions of the miniaturized specimens used were 3 × 4 × 27 mm (thickness × width × length). This specimen type allows utilizing a limited amount of test material or the broken halves of precracked Charpy or larger specimens. The test material comes from the broken halves of 0.5 T SE(B) specimens previously tested for the determination of the reference temperature at Ciemat, Madrid. The fracture toughness tests were performed in the transition region of the steel according to the recommendations of the standard ASTM E1921 and according to Wallin's recommended temperature range of miniaturized specimens. The reference temperature of the Master Curve was very similar to the ones obtained from three-point-bend specimens of sizes 0.2 T, 0.4 T, and 0.5 T. The results obtained confirm a necessity to conduct tests at low temperatures and to test a sufficient number of specimens in order to generate enough valid data for the determination of the reference temperature.