Foam-based warm-mix asphalt technologies decrease the overall viscosity of the asphalt mixture at lower temperatures so that it is workable during construction. However, after construction, viscosity increases rapidly as the foam disappears and the temperature drops. Although the production of the foamed binder is a relatively simple process, in which hot binder is mixed with a limited amount of water (typically 1% to 3% by weight of the binder), the rheology of the foamed binder is complex. The characteristics of foamed binders depend on various factors such as the binder type, grade, and modification; the foaming technology used; amount of water; and temperature. This study investigated the sensitivity of the characteristics of foamed binders to parameters such as injected air pressure and water content. The foamed binder characteristics, including the expansion ratio, half-life, foam index, bubble size distribution, and surface area index, were measured with a novel automated laboratory device called the asphalt foam collapse test. The measurements were conducted in several conditions, including (a) constant water content and variable air pressure, (b) variable water content and constant air pressure, and (c) variable air pressure and water content. It was concluded that both water content and air pressure significantly affected foamed binder characteristics and their internal morphology.