Density and hardness of the negative pastes of lead–acid batteries, to which organic compounds with or without quinone structures have been added, are investigated by means of the stick insertion depth test. Results show that the density and hardness of a paste containing anthraquinone are almost the same as one containing anthracene. By contrast, the two parameters are very different for pastes containing 1,2-naphthoquinone-4-sulfonic acid sodium salt and 1-naphthalenesulfonic acid sodium salt. Commercial lignin derivatives (Vanillex N, Vanisperse A, Indulin AT), which are used as additives for negative plates in lead–acid batteries, are also investigated by means of ultraviolet (UV) spectroscopy. It is found that these lignin derivatives contain quinone structures.