The files in this data set contain monthly mean landfast sea ice data gathered from both Russian Arctic and Antarctic Research Institute (AARI) and Canadian Ice Service (CIS) sources. Data are in the form of percent of area covered, within a longitude-latitude grid with 0.2° resolution. Details on processing and treatment are given in the contributor's PhD thesis (König, 2007). The data are provided in NetCDF format. The time span over which data are available is split into three ranges: for 1953-1967 there are only AARI data, for 1968-1990 both AARI and CIS data are available, and from 1991-1998 only CIS data are available. There are a total of six files: two for the 1953-1967 data, two for the 1968-1990 data, and two for the 1991-1998 data. There are two files for each range because the files with "_noNaN" in their names contain "-1000" as the missing value, and the other files use "nan" as the missing value. Otherwise, the data in those files are identical. König obtained AARI data from NSIDC data set AARI 10-Day Arctic Ocean EASE-Grid Sea Ice Observations. NSIDC has replaced that data set with Sea Ice Charts of the Russian Arctic in Gridded Format, 1933-2006 (AARI, 2007). That data set was edited and compiled by V. Smolyanitsky, V. Borodachev, A. Mahoney, F. Fetterer, and R. Barry (https://nsidc.org/data/g02176). Notice to Data Users: The documentation for this data set was provided solely by the contributor, Christof S. König, and was not further developed, thoroughly reviewed, or edited by NSIDC. Thus, support for this data set may be limited.