The empirical formula of hydrogen peroxide (H2O2) is HO. This is because the molecular formula H2O2 shows 2 hydrogen atoms and 2 oxygen atoms, and simplifying the ratio 2:2 by dividing both by their greatest common factor (2) results in a 1:1 ratio of hydrogen to oxygen atoms. Hence, the empirical formula is HO, representing the simplest whole number ratio of elements in the compound.