Spoleto Festival's marathon chamber music series at the Dock Street Theatre is considered by many patrons an essential event without which the late-May heat and crowded restaurants would be unbearable.

Why bother with Spoleto if you can't secure a seat before that happy coterie, that special genus of artists who configure and reconfigure themselves in order to delight their fans (veritable groupies some of them) with repertoire that ranges from joke to juggernaut?

For one thing, the concerts - two a day for 17 days - are utterly predictable in their unpredictability and their very high quality. The musical selections are astonishingly diverse. You might dislike one piece only to remember the performance of the next one for the rest of your life.

On Wednesday, the gang presented its Program IV, featuring one highlight after another. It began with a gorgeously rendered Violin Concerto in B-flat Major by Pergolesi. Livia Sohn soloed in front of an improvised Baroque chamber orchestra. It was a lovely piece played with pure grace, intelligence and the right dose of sentiment.

Then came Swedish mezzo-soprano Charlotte Hellekant with her collaborator (and festival regular), pianist Pedja Muzijevic, to perform two Swedish songs (Stenhammar, Rangstrom), one American song (Ives) and one English song (Britten).

Hellekant was careful to explain the tunes before singing them, which was helpful, and which gave patrons a taste of how personable and thoughtful she is. Her performance was riveting. Hellekant is a communicator above all else, and she uses her instrument in order to convey, sometimes forcefully, sometimes subtly, the full meaning of her songs. It is obvious that she has internalized all the nuances of text and music.

Next, Geoff Nuttall joined fellow violinist Daniel Phillips, violist Masumi Rostad and cellist David Ying to play Haydn's groundbreaking String Quartet in C Major, op. 20, no. 2, as if determined to reassert Haydn's importance as the inventor of the form.

The concert ended with a cool barnburner called "Techno-parade" by Guillaume Connesson, scored for flute (Tara Helen O'Connor), clarinet (Todd Palmer) and piano (Inon Barnatan).

On Thursday, the "Techno-parade" players were joined by oboist James Austin Smith for a performance of an ecstatic caprice by Saint-Saens. Then Hellekant returned for more heartfelt singing, this time offering two arias from Handel's "Giulio Cesare."

Program V ended with a Mendelssohn's Viola Quintet in B-flat Major, op. 87, played by violinists Daniel Phillips and Mark Fewer, violists Masumi Rostad and Gabriela Diaz and cellist David Ying. It was quite a show.

The quintet, a late work by Mendelssohn, began bright and playful, moved to darker terrain, especially in the brooding 3rd movement, then ended with unbridled joy. It was a vigorous and expressive performance, a display of expert collaboration and genuine exuberance.

Why bother with Spoleto's chamber music series? For this. Again and again.