The aim of this study was to analyze the relationship between the cognitive load when using selected online services and the level of accessibility of these services. The Zara website was selected for the study as a website with a low level of accessibility and the Zalando website as a website with a good level of accessibility. The accessibility of websites was determined on the basis of their compliance with the web content accessibility guidelines (WCAG) using automatic online tools supported by an expert evaluation. The cognitive workload was estimated based on the analysis of the time-domain HRV (heart rate variability) parameters and NASA-TLX inspired questionnaires. In the study participated 5 persons with visual impairments, 4 men and 1 woman, on average 40 years old. The results indicate a significant influence of the level of website accessibility on the mental workload of the users with visual impairments.