The substitution of portland clinker by supplementary cementitious materials (SCMs) is one of the main avenues for the further decarbonization of cement production. To secure future SCM supplies, generic and performance-related test methods for SCMs are required. The International Union of Laboratories and Experts in Construction Materials, Systems and Structures (RILEM) Technical Committee 267-Tests for Reactivity of SCMs (TRM) recommended the so-called R3 test methods for SCM reactivity testing and ASTM adopted the test as ASTM C1897-20, Standard Test Methods for Measuring the Reactivity of Supplementary Cementitious Materials by Isothermal Calorimetry and Bound Water Measurements, in 2020. This paper presents the results of an interlaboratory study (ILS) that was carried out to develop precision statements for the ASTM C1897-20 test methods. The ILS was conducted and interpreted according to ASTM C802-14, Standard Practice for Conducting an Interlaboratory Test Program to Determine the Precision of Test Methods for Construction Materials, and precision statements were prepared following ASTM Construction Materials. The ILS involved the testing of three centrally supplied conventional SCMs by both test methods (R3 hydration heat and bound water) by at least eleven labs. The processing and analysis of the ILS data for single-operator and multilaboratory precision are presented.