The influence of sorption and biotransformation of As compounds and their speciation in rhizosphere and/or in the bulk soil on bioavailability of these compounds is evident. Various analytical methods of determination of As compounds in soil and plants are available for investigation of transformation of As in soil and plants. However, many limitations of these methods were reported. A variety of methods make difficult a comparison of results from individual laboratories, selectivity of extraction agents is frequently insufficient, and the results do not sufficiently reflect changes in experimental parameters. For improvement of credibility of the results, a combination of two or more techniques is recommended. For soil samples, diffusive gradient in thin films or determination of As mobility in soil solution collected during vegetation period are available. For plants, optimization of extraction techniques or application of non-destructive techniques such as X-ray spectrometry and laser ablation are necessary in further research.