<p>One of the major difficulties for reliably assessing the durability of salt laden building materials is the measurement of the in-pore salt crystals content without damaging the materials, which are frequently of great historical value. In this contribution, it was attempted to estimate the content of sodium sulfate decahydrate (mirabilite) by the combination of two methods: differential scanning calorimetry and ultrasonic pulse velocity. The proposed approach allowed to develop the relation between the P-wave velocity (non-destructive) and in-pore salt crystal content. Fully saturated red clay brick and sandstone with 10%, 15% and 20% Na₂SO₄ solutions were investigated. The idea was to first evaluate the content of crystallized in-pore mirabilite in the noted materials via differential scanning calorimetry. Subsequently, the content of in-pore mirabilite was related to the P-wave velocity increase that it caused with respect to the initial empty (no salt) dry state. Regardless of the anisotropic microstructure of the red clay brick, linear relations were obtained between the content of crystallized mirabilite and the P-wave velocity increase in all directions. The slope of such linear relations was higher in the direction which had a lower initial dry P-wave velocity. Linear relation was also obtained in the case of the isotropic sandstone, however, with a lower slope than all the directions of the red clay brick. Accordingly, the proposed non-destructive approach showed promising potential for the monitoring and preservation of the porous building materials which are susceptible to salt crystallization damage.</p>