Compatibility has recently been conceived as the degree to which partners either facilitate or interfere with each other's interaction goals. Compatability is thus perceived as an on-going process, rather than an end state. A study was conducted to develop an index of compatibility based on pair interaction, where compatibility is conceived as the degree to which partners facilitate each other's goals (Levinger & Rands, 1985). For two semesters, 13 female roommate pairs weekly reported their two most important relationship interactions and filled out a variety of questionnaires assessing similarity, conflict, emotional involvement, satisfaction, and leisure time. A Facilitation/Interference (F/I) Ratio was created from ratings of the weekly events to represent roommates' compatibility, and this index was compared to two other measures of compatibility, Demographic Similarity and Personal Similarity. By second semester, compatible roommates, based on the F/I Ratio and Personal Similarity, reported their conflicts to be fewer in number, less frequent, less important, and less intense than did incompatible roommates. They also reported fewer unilateral problem-solving attempts, more satisfaction with the roommate, and more leisure time spent together than did incompatible roommates. (Questions regarding the measure and method are explored and future directions suggested.) (Author/NB)