The study analyzed the results of open time measurements of ceramic tile adhesives after not less than 30 minutes, obtained by 27 laboratories participating in the interlaboratory comparison program. The obtained results were statistically analyzed following the requirements of ISO 13528 and assessed under the z-score analysis criteria contained in the EN ISO/IEC 17043. 22 out of 24 laboratories that sent the results of the open time measurement confirmed their competencies, obtaining a “satisfactory” result following the evaluation criteria. The result obtained by one laboratory was classified as “questionable”, and another received the result as “unsatisfactory”. From the perspective of laboratories participating in interlaboratory comparisons, the obtained results should be assessed as satisfactory, and thus the laboratories confirm their competence. However, a wide scatter of the results of the open time measurement should be noted – 22 laboratories, the results of which were classified as “satisfactory”, obtained results from 0.1 N/mm2 to 1.0 N/mm2. Such a wide scatter of the results of open time measurements determined by the adhesion strength indicates that this method has too little accuracy, and it should not be used in the assessment and verification of constancy of performance of ceramic tile adhesives.