- Sandblasting, Thornhill - General Contractors
- Sandwich Shop Restaurants, Thornhill
- Scientific Equipment, Thornhill
- Scientific Services, Thornhill
- Seafood Restaurants, Thornhill
- Secondary Schools, Thornhill
- Security Brokers & Dealers, Thornhill
- Septic Tank Pumping & Cleaning, Thornhill - Septic Systems
- Service & Gas Station Equipment & Services, Thornhill
- Service Station Equipment - Installation, Repair & Maintenance, Thornhill
- Sewer & Drain Cleaning & Repair, Thornhill - Drain Cleaner & Installation Services
- Ship & Boat Building & Repairing, Thornhill
- Shopping & Shopping Services, Thornhill
- Siding Materials, Thornhill
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- Snowmobile Dealers, Thornhill - Parts, Gear, Clothing & Accessories
- Social Services & Welfare, Thornhill
- Societies & Foundation Associations, Thornhill
- Solar Equipment, Thornhill
- Soup & Salad Restaurants, Thornhill
- Special Coatings, Thornhill - Protective Coatings
- Special Interest Schools Or Instruction, Thornhill
- Specialty Stores, Thornhill
- Sporting Goods Stores, Thornhill
- State Government, Thornhill
- Stereo, Audio & Video Equipment Dealers & Stores, Thornhill
- Stock & Bond Brokers & Dealers, Thornhill
- Stocks & Bond Brokers, Thornhill
- Submarine Sandwich Restaurants, Thornhill
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