Existing empirical studies of the degree of sustainability among various monetary assets generally ignore several financial instruments which only recently have become prominent. Here this deficiency is remedied by investigating substitution behavior within a portfolio of near-monies which includes money market funds and interest bearing check able deposits, as well as conventional savings and time deposits. The research uses a standard technique of empirical production studies that has recently been applied to the issue of monetary asset substitution by Sims, Takayama, and Chao (1987). The findings here provide evidence of significant sustainability between each of the "modem" near-monies examined and money narrowly defined.