Scalability is one key concept for analyzing the performance of parallel systems, specially in clusters and Grid environments. Even though heterogeneous systems are becoming more common, there is no suitable definition of this concept for studying their behaviour. This paper presents an extension of the isoefficiency method that permits its application to heterogeneous environments as well. The heterogeneous isoefficiency function can predict the system scalability independently of the algorithm nature. This method is based only on the algorithm's overhead time and an adequate workload distribution. This paper also presents a set of theoretical scalability analyses, and finally some experiments where the method's usefulness and validity are shown.