Abstract: There is provided substantially crystalline forms of 4-({3-[7-(3,3-dimethyl-2-oxobutyl) -9-oxa-3,7-diazabicyclo[3.3.1]non-3-yl]propyl}amino)benzo-nitrile; tert-butyl 2 -{7-[3-(4-cyanoanilino)propyl]-9-oxa-3,7-diazabicyclo-[3.3.1]non-3-yl}ethylcarbamate; tert-butyl 2-{7-[4-(4-cyanophenyl)butyl]-9-oxa-3,7-diazabicyclo[3.3.1]non-3-yl}ethylcarbamate; or tert-butyl 2-{7-[2S)-3-(4-cyanophenoxy)-2-hydroxypropyl]-9-oxa-3,7-diazabicyclo-[3.3.1]non-3-yl}ethylcarbamate and pharmaceutically-acceptable salts thereof, which compounds are useful in the treatment of cardiac arhythmias.