The influence of concentration and nature of the acid, type and tuning of RF-generator, method of solution introduction (flow injection or two-nebulizer method) are investigated to understand the nature of acid matrix interferences in ICP-AES. Sixteen elements and nine different spectrometers wer used to study the matrix interferences caused by HCl, HNO3, HClO4, H2SO4, H3PO4. There are two effects connected with acid: depression of the analyte emission signals and irregular signal changes for different elements. The values of these effects depend on the RF-generator type. The obvious results of introducing acid in the plasma are a decrease in plasma temperature and departures from LTE.