Deoxynojirimycin and deoxygalactonojirimycin derivatives according to the present invention are N-alkylated D-galacto, D-gluco- or L-ido-deoxynojirimycin with a linear methyloxypentyl group bearing various sidegroups and a non-fused bicyclic aromatic group (“X”) on the methyloxy-carbon. These compounds display an increased inhibitory potency towards GCS, and / or an increased inhibitory potency towards GBA2, and / or a decreased inhibitory potency towards GBA1, relative to known deoxynojirimycin derivatives of the same (D-gluco, L-ido or D-galacto) configuration. Therefore, compounds of the present invention are effective in the treatment of diseases which are associated with an irregular level of cytosolic or lysosomal glucosylceramide and / or higher glycosphingolipids, such as a lysosomal storage disorder, such as Gaucher disease, Fabry disease, Tay-Sachs disease, Sandhoff disease, GM1 gangliosidosis, Sialidosis, Niemann Pick disease type C and AMRF, or a symptom of one of the diseases collectively classed as metabolic syndrome, such as obesity, insulin resistance, hyperlipidemia, hypercholesterolemia, polycystic kidney disease, type II diabetes and chronic inflammation, or a neurodenegerative disorder, such as Parkinson disease or Lewy-body dementia.