Miscibility  /mɪsɨˈbɪlɨti/ is the property of liquids to mix in all proportions, forming a homogeneous solution. In principle, the term applies also to other phases (solids and gases), but the main focus is usually on the solubility of one liquid in another. Water and ethanol, for example, are miscible since they mix in all proportions.

By contrast, substances are said to be immiscible if in some proportion, they do not form a solution. For example, diethyl ether is fairly soluble in water, but these two solvents are not miscible since they are not soluble in all proportions.

In organic compounds, the weight percent of hydrocarbon chain often determines the compound's miscibility with water. For example, among the alcohols, ethanol has two carbon atoms and is miscible with water, whereas octanol with a C₈H₁₇substituent is not. Octanol's immiscibility leads it to be used as a standard for partition equilibria. This is also the case with lipids; the very long carbon chains of lipids cause them to almost always be immiscible with water. Analogous situations occur for other functional groups. Acetic acid (CH₃COOH) is miscible with water, whereas valeric acid (C₄H₉COOH) is not. Simple aldehydes and ketones tend to be miscible with water, since a hydrogen bond can form between the hydrogen atom of a water molecule and the unbonded (lone) pair of electrons on the carbonyl oxygen atom.