## 1 Introduction  
In times of increased exposure of the body to highly reactive forms of oxygen and nitrogen, the innate defense mechanisms are not sufficient to neutralize them effectively. This leads to their excessive accumulation in the body and the formation of so-called oxidative stress, which promotes damage to cellular DNA, protein molecules, fatty acids, and cell membranes. As a result, cell and tissue dysfunction occur, and many diseases develop, including cancer [1]. One of the relatively simple ways to combat oxidative stress is to ensure a balanced diet [2]. Nature provides many substances with antioxidant properties [3]. These include products rich in polyunsaturated fatty acids, fiber, and polyphenols [4]. Research conducted especially on the latter has shown that they protect cellular components from oxidative damage and help prevent degenerative diseases [5]. Taking the above into account, research is being conducted worldwide on effective methods of isolating polyphenic compounds from natural sources. The importance of this topic is enhanced by the fact that for many of them, chemical synthesis is an unprofitable undertaking, among others, due to the diversity and complexity of their structure. It is also important that many polyphenols are unstable compounds, and during extraction from plant material, they can be transformed/degraded into other substances, often with unknown biological effects on the human body. As a result, the efficiency of isolation of a given group of compounds is the result of two parallel and opposing processes, i.e., the release of compounds from the plant matrix and their degradation. Both depend on the extraction conditions used. Currently, various approaches are used to isolate compounds from plants. These include simple techniques useful on a laboratory scale for assessing the actual content of antioxidants in plant material, as well as more advanced ones, consistent with the principles of green chemistry, which can be used in the industrial isolation of natural compounds. The latter, commonly referred to as \"assisted extraction techniques\", are usually more convenient to use, more economical, more environmentally friendly, and faster. One such technique is ultrasound-assisted solvent extraction (UASE), in which the effects related to the cavitation phenomenon increase the extraction efficiency per unit of time [6]. This work is part of the trend of research on natural polyphenic compounds. Its main goal is to find the most effective conditions of resveratrol extraction from Japanese knotweed (_Polygonum cuspidatum_), ensuring high antioxidant activity of the extract, thus extending the data discussed in [7]. Japanese knotweed is considered in many countries, especially in Europe, to be not only an invasive species, but also expansive and therefore undesirable in the natural environment [8]. It is recommended to remove it before the flowering period and then mechanically destroy it because it grows back easily. Nevertheless, despite its bad reputation, it is a valuable source of polyphenic compounds and especially resveratrol [7]. This compound has many health-promoting properties, including preventing the development of serious diseases [9; 10]. For this reason, it is highly desirable in many industries. On an industrial scale, it is typically obtained from dried red grape cuitics. Thus, finding effective conditions for isolating resveratrol from knotweed will allow the use of this undesirable weed as an alternative source of this valuable compound for the production of supplements and food additives, contributing to balanced development and increasing the ecological and economic benefits of many economies. In this study, UASE was used to prepare extracts, a technique that can be easily scaled up to industrial needs while respecting pro-ecological principles. As extraction solvents, in place of expensive ethanol, its substitute, i.e., methanol, water, and a mixture of equal volumes of these two components, was used.