A cationic water-soluble pillar[5]arene has been synthesized and revealed to encapsulate linear acids in neutral, alkaline, and acidic conditions, driven by the hydrophobic and hydrogen bond interactions. A higher guest hydrophobicity contributes to a higher host affinity. Methanoic acid (A1) and acetic acid (A2) do not show any affinity for the host at all, because they are too hydrophilic for the host to compete with water.