Quick verdict
| Use case | Pick |
|---|---|
| Natural / clean-beauty positioning | Glycerin (consumer-trusted, plant-derived) |
| Dissolving hard-to-dissolve actives (salicylic acid, retinoids) | Propylene Glycol (better solvent) |
| Hydrating face serum at 3-5% | Glycerin (deeper hydration, better tolerated) |
| Penetration-enhanced active serum | Propylene Glycol (boosts active delivery) |
| Sensitive-skin product line | Glycerin (lower sensitisation) |
| Budget mainstream lotion | Either — depends on positioning |
Why both exist
Both are small polyol humectants and solvents:
- Glycerin (1,2,3-propanetriol) — 3 OH groups, heavier molecule. Plant-derived (saponified vegetable oils) or fermented. Sweet taste, sticky at high concentrations.
- Propylene Glycol (PG) (1,2-propanediol) — 2 OH groups, smaller and lighter molecule. Synthesised from propylene oxide. Slightly bitter, less sticky.
Both are humectants (bind atmospheric water to skin) and solvents (dissolve water-soluble actives). PG is also a penetration enhancer — it transiently disrupts the stratum corneum lipid matrix to help actives reach deeper layers.
When Glycerin wins
- Consumer perception — “natural” plant-derived label.
- Long-term hydration — heavier molecule sits on the skin surface longer.
- Sensitive skin — lower sensitisation rate than PG.
- Pregnancy/baby products — universally accepted as safe.
- High-percentage humectant — at 5% glycerin gives more hydration than 5% PG.
When Propylene Glycol wins
- Difficult-to-dissolve actives — salicylic acid, some retinoids, many botanical extracts.
- Penetration enhancement — improves delivery of actives into deeper skin.
- Light feel — less sticky than glycerin at the same concentration.
- Fragrance/extract solvent — dissolves a wider range of aromatic compounds.
- Mild preservation boost — at 5%+ slightly reduces water activity.
The stickiness problem
Glycerin above 7% in a leave-on can feel tacky/sticky, especially in humid climates. PG is much less sticky at the same concentration — but lighter humectant effect.
Trick: glycerin + PG together at 3-5% each gives strong hydration without the heavy stickiness of 8% glycerin alone.
The “PG is antifreeze” myth
Propylene glycol (food/cosmetic grade) is distinct from ethylene glycol (the toxic automotive antifreeze ingredient). PG is GRAS for food use, common in pharmaceuticals, and has decades of cosmetic safety data. The “PG = antifreeze” confusion is a common myth.
That said, some customers actively avoid PG. For these audiences, use Propanediol (1,3-propanediol, plant-derived from corn fermentation) — similar function to PG, natural-positioned label.
Usage rates
- Glycerin: 2-10% (sweet spot 3-5%)
- Propylene Glycol: 1-15% (sweet spot 2-5% as humectant, 5-15% as solvent)
Substitutes
- Propanediol — natural-positioned alternative to PG.
- Butylene Glycol — fellow small-molecule humectant, between glycerin and PG in feel.
- Pentylene Glycol — humectant + mild preservative-boost.
- Sodium PCA — lighter humectant, much less sticky.
- Sodium Lactate — humectant + mild keratolytic.
→ Full ingredient page: Glycerin · Propylene Glycol