A monomethod bias still prevails in the psychology of religion, with the developing field studying the relationship between religiosity, spirituality and health being almost completely dominated by questionnaire research. This comes as a surprise, because the experiential side of religion, spirituality, can by definition be regarded as inner and private experiences of transcendence that have frequently been described as being of utmost importance. At first glance, from this perspective, standardized questionnaire scales appear to be inappropriate for “measuring the unmeasurable”. Until (...) now, no questionnaire instrument developed for the purpose of assessing spiritual and religious experiences has been qualitatively double-checked for intermethod validity. In order to throw light on whether questionnaire instruments are appropriate for measuring domains of spiritual and transcendental experiences, we tested a newly developed questionnaire instrument, the Exceptional Experience Questionnaire , for intermethod validity by conducting post-questionnaire interviews . This revealed that the EEQ was able to grasp these experiences within reasonable limits. However, two thirds of our interview sample said that the study had changed how they viewed these experiences. These findings suggest that the reactivity of quantitative instruments in grasping spiritual and transcendental domains should be taken into account, especially when devising longitudinal designs and further research is clearly needed. As a consequence, introducing a reflective circle of results stemming from both quantitative and qualitative approaches is recommended as the most useful and appropriate research tool for studying spiritual or mystical experiences, if not all constructs. (shrink)