Improved separation of highly toxic contact herbicides paraquat (1,1′-dimethyl-4-4′-bipyridinium), diquat (6,7-dihydrodipyridol[ 1,2-a:2′,1′-c]pyrazine-5,8-di-ium), difenzoquat (1,2-dimethyl-3,5-diphenyl-1H-pyrazolium-methyl sulfate), mepiquat (1,1-dimethyl-piperidinium), and chloromequat (2-chloroethyltrimethylammonium) were presented by high-performance thin-layer chromatography (HPTLC). The quantification is based on a derivatization reaction, using sodium tetraphenylborate. Measurements were made in the wavelength range from 500 to 535 nm, using a light-emitting diode (LED) for excitation purposes, which emits very dense light at 365 nm. For calculations, a new theory of standard addition method was used, thus leading to a minimal error if exactly the same amount of sample content is added as a standard. The method provides a fast and inexpensive approach to quantification of the five most important quats used for plant protection purposes. The method works reliably because it takes into account losses during pre-treatment procedure. The method meets the European legislation limits for paraquat and diquat in drinking water according to United States Environmental Protection Agency (US EPA) method 549.2 which are 680 ng L−1 for paraquat and 720 ng L−1 for diquat. The method of standard addition in planar chromatography can be beneficially used to reduce systematic errors. Although recovery rates of 33.7% to 65.2% are observed, calculated contents according to the method of standard addition lie between 69% and 127% of the theoretical amounts.