The present chapter calculates the liquidity usage of the Mexican Real Time Settlement Payment System, SPEI, during a one month period. In particular, the authors’ interest is to give insight about how payment systems participants manage the settlement of low and large value payment transactions in real-time. To that end, they create an artificial environment, in which they use historical data from April 7 to May 7, 2010, and reproduce the operational conditions of SPEI. All payment instructions that arrived from 9:00 a.m. to 17:00 p.m. each working day are included. For each of these days, the authors arrange the transactions in four sets, delimited according to their value: all payments; payment orders with value higher than 100,000 MXN; transactions with value higher than 1,000,000 MXN; and payments with value higher than 10,000,000 MXN. The authors use a simulation environment to reproduce SPEI’s operational conditions in order to measure the liquidity usage in different settings of settlement speed requirements.