In this paper, we present the design and evaluation of a nearfield communication-based mobile p2p payment application called mFerio which is designed to replace cash-based transactions. We first identify design criteria that payment systems should satisfy and then explain how mFerio, relative to those criteria, improves on the limitations of cash-based systems. We next describe mFerio's user interface design and implementation, focusing on the balance between usability and security. Finally, we present the results of a two-phase user study, involving a total of 104 people, that shows that mFerio is fast, accurate, easy to use, with low cognitive load ? even outperforming cash in common situations. ACM Classification Keywords H.5.2 User Interfaces: Mobile Payment Systems General Terms Design, Human Factors, Experimentation Author Keywords Mobile Computing, User Study, Mobile Payment, p2p Payment, NFC, Near-Field Communication, Digital Wallet