In deep penetration laser welding, the keyhole constitutes the primary source of the process dynamics causing defects or quality deficiencies in the resulting weld seam. Although the keyhole has been subject to extensive research over the course of the past decades, existing models still are not entirely capable of predicting the process dynamics reliably. A major reason for these shortcomings of the existing models is the insufficient consideration of the temperature dependencies of material properties mostly due to the difficult experimental accessibility of the keyhole. Thus, gaining access to physical properties of the keyhole is crucial to validate and further develop existing models. In previous studies, the method of measuring temperatures at the keyhole front wall by using small tantalum probes as refractory channels for pyrometric measurements has been introduced. In this research, the method of refractory probes is employed to measure the axial temperature distribution of pure aluminum, nickel and copper under low vacuum. The comparability between the different materials was established on the basis of an equal weld depth. For comparison, the spatially resolved temperature field in the region of the keyhole opening was calculated additionally based on high-speed video recordings. It was found that under reduced ambient pressure the keyhole wall temperatures are lowered significantly compared to atmospheric pressure but still exhibit overheating in the magnitude of a few hundred Kelvin. Furthermore, significant qualitative differences in the axial temperature distributions between the materials could be identified which are attributed to different absorption properties regarding the laser wavelength.