When a file is loaded UltraEdit checks the first 7K of the file and if it contains more than two NULL (Hex 00) the file is considered a binary/HEX file. NULLS are not valid in an ASCII file and will be converted to spaces if you edit the file in text mode. If you view a HEX file in text mode but make no changes UltraEdit should not convert the NULLS to spaces, but if you do make changes in TEXT mode to a HEX file the change will occur. If you go to Advanced -> Configuration -> Editor -> Advanced and select “Allow editing of text files with HEX 00’s …” you should be ok. For UltraEdit prior v11.20 this setting is on the General tab toward the bottom under “Advanced/Problem Solving”.