The underlying concept of the standard effectiveness-number of transfer units (NTU) model is that the effectiveness of an exchanger can be correlated to two dimensionless parameters, namely, heat capacity ratio (Cr) and NTU. However, a limitation of this model is that it cannot account for the changes in effectiveness due to changes in operating temperature and humidity of simultaneous heat and moisture exchangers, specifically liquid-to-air membrane energy exchangers (LAMEEs). The purpose of this paper is to explain the reason for this limitation and also to explore the extension of the aforementioned concept of the effectiveness-NTU model to LAMEEs. The first contribution of this paper is to demonstrate that the reason for this limitation is that one of the simplifying assumptions of the standard effectiveness-NTU model, i.e., that Cr represents the ratio between the changes in the temperatures of the two fluid streams across an exchanger, is not applicable to LAMEEs. Further analysis in this paper yields two new fundamental dimensionless parameters that are analogous to Cr, termed effective Cr and effective m*, which represent the actual ratios between the changes in the temperatures and humidity ratios of the fluid streams. Then, it is shown that models analogous to the standard effectiveness-NTU model can be used to correlate the dependency of the effectiveness of LAMEEs on the operating temperature and humidity to effective Cr and effective m*.