In this paper we inquire material qualities for tangible user interfaces and introduce the term “ephemeral user interface”. We present an example user interface that employs fragile soap bubbles for human-computer interaction: the user has to move or blow bubbles over a dark liquid surface in order to interact with a computer system. Our installation uses liquid, smoke, and soap bubbles as unusual and transient materials that demand a very cautious and calm handling, and furthermore elude the complete control of users, as the bubbles inevitably burst after a short while. Thus, the soap bubble interface provokes thoughts about durability, control, and materiality of tangible user interfaces. Author Keywords Ephemeral user interfaces, tangible user interfaces, materiality, soap bubbles, interactive art installation ACM Classification Keywords H.5.2 Information interfaces and presentation (e.g., HCI): User Interfaces – Input devices and strategies General Terms Design, Human Factor...