This paper aims to improve our understanding of demonstrators' atmosphere perceptions, that is, demonstrators' affective state, which is induced by the protest environment. We examined how demonstrators perceive protest atmosphere, why they do so, and whether atmosphere perceptions influence demonstrators' future collective action preparedness. We hypothesized that demonstrators' atmosphere perceptions diverge on a dimension of pleasure, and relate to their grievance (i.e., perceived societal intolerance), group identification, empowerment, and perceived police aggression. A pleasant atmosphere perception was expected to stimulate a demonstrator's future action preparedness. We tested these hypotheses with a mixed-methods dataset of two Dutch protests, staged by Lesbians, Gays, Bisexuals, and Transgenders and anti-monarchists. Our analyses revealed that demonstrators' atmosphere perceptions diverge on a dimension of pleasure, and relate to group identification, empowerment, and, for anti-monarchists, perceived societal intolerance. A pleasant atmosphere perception deters a demonstrator's future action preparedness and also stimulates his or her group identification and empowerment, which, then, stimulate his or her action preparedness.