He gave freely to charitable works, founding an orphanage and a scientific institute in Moscow and a school of commerce in Saint St. Petersburg, as well as financing the opera in Saint St. Petersburg and hundreds of people"s schools and philanthropic institutions across Russia. His eccentricities were famous and, because he thought he had been deceived by British merchants during a stay in England, one day he bought all the available hemp to "teach the English a lesson". Even so he managed to increase the family fortune so that, on his death, he owned 55 foundries and metallurgical factories.
His tomb is in the Donskoy Monastery.
Demidov was interested in botany, wrote a treatise on bees, collected a herbarium, and had a number of birds caged in his house. A catalogue of his plants was prepared by Peter Simon Pallas.
His manuscript of Kirsha Danilov"s folk songs is preserved in the Russian National Library.