The law of multiple proportions states that if two elements can combine to form more than one compound, the masses of one element that combine with a fixed mass of the other element, are in the ratio of small whole numbers.

Examples of this law include (i) Hydrogen combines with oxygen to form two compounds, water and hydrogen peroxide.

2H₂ + O₂→ 2H₂O

H₂ + O₂→ H₂O₂

In this example, the ratio of masses of oxygen which combine with the fixed mass of hydrogen is 16:32 or 1:2, which is a simple whole number ratio.

(ii) Carbon and oxygen combine to form carbon dioxide and carbon monoxide.

C + O₂→ CO₂

2C + O₂→ 2CO

In this example, the ratio of masses of oxygen which combine with the fixed mass of carbon is 16:32 or 1:2, which is a simple whole number ratio.

The law and the examples of the law show that in the formation of a compound, there are constituents which combine in a fixed proportion. The constituents may be atoms. Thus this law confirms the existence of atoms which combine to form molecules and compounds.