The archiving of the web was demonstrated and the variability of the archived web pages was researched in the web archive of the National and University Library in Ljubljana (SANUK). The goal of the examination was to determine the length of the lifespan of the sources, that are included in this archive, as well as to investigate, how these resources vary. The research is based on the examination entitled Lifecycle and variability of web resources in Croatian web archive (Životni vijek i promjenjivost građe hrvatskog arhiva weba), carried out by Holub, Getliher and Rudomina (2011) within the Croatian web archive. Included in the research were both browsing through the captured web pages of the web archive of SANUK and examining of these. The research took place from 1.5.2016 till 16.5.2016, namely on those web pages, that were archived since the time period mentioned. With 353 randomly chosen web pages (27% of 1320) the following was examined: what is the frequency of the page acquisition, what is the mean lifespan, how many of the captured web pages are no longer available on the live web, were there any changes regarding the URL title, page title and the ownership of the page as well as what changes were detected in the formation of these pages. It was discovered that on the live web 12,46% of web pages in the sample are no longer available, that the URL title changed in 11,05% of web pages, that the title changed in 14,72% of web pages, that the ownership changed only in 2,83% of web pages and that the formation changed in 14,37% of web pages from the sample. The captured web pages generally represent an important part of our cultural heritage. Their importance is established for the researchers as well as individuals. With this research it was confirmed that the Slovenian web pages are delicate, variable and transient. Upon further in-depth examination of the content of SANUK our results could be confirmed and the reasons would be revealed that define the importance of the functioning of archives of the world wide web.